INVOKING THE GREEKS ON THE RELATIONBETWEEN THOUGHT AND REALITY:TRENDELENBURG’S ARISTOTLE—NATORP’S PLATO1

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I. INTRODUCTION

My interest in the Neo-Kantian reception and use of Greek philosophy, andespecially Paul Natorp’s reading of Plato’s theory of forms in his monumentalPlatos Ideenlehre (1903), is rooted in my primary interest, which is the philosophyof Plato and Aristotle, and the present article is written largely from this perspec-tive. My one question, therefore, is whether Natorp’s reading can contribute to ourunderstanding of Plato. I shall argue, perhaps unusually, that it can. I do not meanthat we can accept it in general and as it stands but rather that it is a reading weneed to take seriously when considering certain questions that we would agree arecentral for the understanding of Plato. The questions I have in mind include:

Q1. How are Plato’s forms related to explanations (aitiai)?Q2. How are Plato’s forms related to sense-perceptible things?Q3. How are Plato’s forms related to thought and knowledge?

That we need to give serious consideration to Natorp’s reading when addressingthese questions can be shown only through close attention to particular Platonictexts, and I think it can be shown if we concentrate on such texts as, first, theaccount of explanation in the Phaedo, when addressing questions 1 and 2, andsecond, the argument in the Sophist for the reality of the two central “mostimportant/largest kinds” (megista gene), change and changelessness (kinesis andstasis), when addressing question 3.

1 For helpful comments, I would like to express my thanks to Gail Fine, Paul Guyer, Frans de Haas,Terry Irwin, Pauline Kleingeld, Eric Schliesser, Abraham Stone, and Daniel Watts.

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My other question is whether Adolf Trendelenburg’s criticism of Kant and hispositive and systematic defense of a distinctive alternative to transcendental ide-alism can contribute to our understanding of Aristotle. Trendelenburg was aleading historian of ancient philosophy and Aristotelian scholar, and this may inpart explain why he kept his philosophical as opposed to historical work, and inparticular the Logische Untersuchungen (1840, 3rd ed. 1870) on which I shall berelying, largely free from major explicit reference or appeal to Aristotle. I believe,however, that his criticism of Kant and his peculiar alternative to transcendentalidealism go back to his study of Aristotle. This is certainly how Hermann Cohenunderstood Trendelenburg’s criticism, that is, as basically Aristotelian.2 Trende-lenburg’s positive position, which he defends at great length in the LogischeUntersuchungen, may appropriately be described as a form of transcendentalrealism, or perhaps better, transcendental causal realism, and his particular criti-cism of Kant—the famous charge of fallacy—needs to be situated within hisgeneral and systematic defense of this position. But this position, transcendentalcausal realism, I want to argue, is one to which we need to give serious consid-eration when we ask,

Q4. What, according to Aristotle, is the relation between logic and meta-physics or between the principles of thought and the principles ofreality?

I shall defend this claim by concentrating on Aristotle’s defense of the principle ofnon-contradiction (PNC) as a metaphysical principle in Metaphysics Gamma.

The juxtaposition of Trendelenburg and Natorp is not immediately obvious, andTrendelenburg is not of course a Neo-Kantian. However, I doubt that we canproperly understand the origins of Marburg Neo-Kantianism, and especiallyCohen’s Kants Theorie der Erfahrung, without central reference to Trendelen-burg’s challenge. The overall argument and dialectic between Trendelenburg andCohen (and Natorp following Cohen) can, I think, be understood as follows.Central to the critical philosophy of Kant is the transcendental premise (as we maycall it), which says that certain of our representations or concepts, including thoseof space and time, are necessary conditions for the possibility of objective knowl-edge, and that this can be established a priori and only a priori. But in Kant thispremise is crucially used to argue that our knowledge can only be of appearancesand not of things in themselves; that is, our knowledge can only be of thingsconsidered in relation to the possibility of our knowing them. Our knowledgecannot, in this sense, be objective without qualification; for it cannot be of thingsconsidered in themselves and without any reference to us and our knowledge.

2 See Hermann Cohen’s Kants Theorie der Erfahrung, 1st ed. (Berlin: Dümmler, 1871) esp. 33f.

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Now Trendelenburg, although not a follower of Kant, accepts the transcendentalpremise, and this is clearly important for understanding the overall argument anddialectic between Trendelenburg and Cohen. However, Trendelenburg’s challengeis, first, to argue that the transcendental premise is compatible with the claim thatour knowledge is unqualifiedly objective and of things in themselves, and second,to defend a position that combines the transcendental premise with unqualifiedobjectivity. His basic argument is that we must represent what we experience asbeing, for example, in space and time because what we experience is, in itself andwithout reference to us or our knowledge, in space and time—where this“because” is basically causal.

Trendelenburg, Cohen, and Natorp all share the commitment to the transcen-dental premise. Furthermore, and most striking, Cohen and Natorp share withTrendelenburg the view that the transcendental premise can be accepted only if itcan be shown to be compatible with the claim that our knowledge can be unquali-fiedly objective. I am inclined to think that on this point Cohen and Natorp aredirectly and decisively indebted to Trendelenburg and his challenge. The disagree-ment between Trendelenburg and Cohen–Natorp is about how it is possible todefend the possibility of unqualifiedly objective knowledge in conjunction withthe transcendental premise. Trendelenburg defends a causal realist answer, andthis is just what Cohen and Natorp reject. Their answer, following Kant, is that itis because certain of our representations and concepts are a priori conditions forthe possibility of objective knowledge that they are true of the objects of ourknowledge. Against Kant, however, they argue that no distinction can be upheldbetween an object in itself and an object of possible knowledge, knowledge ofwhich we are in principle capable, and from this they infer that these sameconditions are also true of things without qualification and in themselves.

How, in general, can this debate between Trendelenburg and Cohen–Natorpcontribute to our understanding of Plato and Aristotle? Natorp argues that tran-scendental arguments, arguments that seek to establish the fundamental nature ofreality by relying on the conditions of thought and knowledge, occupy a centralplace in Plato’s philosophy. He argues further that the relation between reality andthought/knowledge in Plato is not primarily causal; the knowledge of forms is notacquired either by the forms directly imprinting themselves on the intellect or bythis knowledge being in some way derived from impressions of sensible things onour senses. How, then, he asks, can we know that there are forms, according toPlato? His answer is that, according to Plato, we establish a priori that knowledgerequires forms, and we derive from this that things, things without qualificationand without reference to us or our knowledge, must fundamentally include forms.At the end of the article I shall take up for consideration and assessment thiscentral argument of Natorp’s. He concludes that Plato is a transcendental idealist,indeed the original and authentic transcendental idealist—and this label may

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naturally grate. The labeling, however, is not what matters, and in any case weought to keep in mind that for Natorp (as for Cohen) transcendental idealism iscompatible with, and indeed supposed to account for, unqualifiedly objectiveknowledge.

With regard to the understanding of Aristotle, our engagement with Trendelen-burg can contribute in the following way. I think we would generally agree, as doTrendelenburg, Cohen, and Natorp, that Aristotle conceives of the relationbetween reality and thought/knowledge in causal realist terms, where this causalrealism is based on his distinctively causal theory of sense-perception. But thismeans that if we are accustomed to associate the transcendental premise, andtranscendental arguments in general, with the denial of unqualified realism, thenwe shall either be predisposed to deny that transcendental arguments can play anyrole in Aristotle, or, if we think that we encounter them, to wonder whether thisdoes not after all imply some questions about his realism. If, however, we takeseriously Trendelenburg’s combination of the transcendental premise with causalrealism, we may be more open to recognize the presence of transcendental argu-ments at certain central junctures in Aristotle’s metaphysics and in particular inhis account of the relation between logic, or the principles of thought, andmetaphysics, or the principles of reality.

Let me briefly clarify the notion of “transcendental argument” in question here.As I am using the term, this is an argument that seeks to establish a conclusion thatis about things from premises that are about our knowledge of things and inparticular the possibility of such knowledge. Whether “things” here signifies onlythings considered in relation to our knowledge of them or it can also signify thingsin themselves and considered independently of our knowledge of them is, we shallsee, a contested issue, which it is better not to prejudge. This notion of a tran-scendental argument is certainly central in Kant. It is most clearly present in thesection of the first Critique entitled “On the Highest Principle of All SyntheticJudgments” (A154/B193f), and it is epitomized in the sentence: “[T]he conditionsof the possibility of experience in general are likewise conditions of the possibilityof the objects of experience, and for this reason they [the conditions of thepossibility of experience] have objective validity in a synthetic a priori judgment”(A158/B197, original emphases).3 It is also the notion of a transcendental argu-ment that is involved in the dialectic between Kant, Trendelenburg, and Cohen–Natorp. Moreover, this notion of a transcendental argument is not only sufficientlyspecific to be recognizably Kantian but also sufficiently broad to allow for andindeed invite such cross-historical comparisons as whether Plato and Aristotleemploy transcendental arguments.

3 See also A111: “The a priori conditions of any possible experience in general are likewise conditionsof the possibility of the objects of experience.”

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It may be objected that Kant’s notion of a transcendental argument is morenarrow than this, because, for Kant, this is an argument that seeks to establish aconclusion that is about things from premises that are about our a priori knowl-edge of things and in particular the possibility of such knowledge—with theemphasis squarely on knowledge of things that is a priori. But it is hard to see whythe fact that Kant is concerned especially with the possibility of a priori knowl-edge of things should constitute an objection to a broader notion of a transcen-dental argument, whose premises are about the possibility of objective knowledgein general. For it can hardly be denied that Kant’s concern with the possibility ofa priori knowledge of things is crucially part of a broader concern with thepossibility of objective knowledge in general. In any case, the objection can beaccommodated; for the knowledge whose possibility is at issue when we considerwhether Plato and Aristotle employ transcendental arguments, such as the knowl-edge of forms or of the objective validity of the PNC, is, if anything, a priori.

II. TRENDELENBURG AND ARISTOTLE

II.1. Trendelenburg’s Response to Kant

Kant defended the transcendental premise, especially with regard to the repre-sentations of space and time, and he used this premise to infer that our knowledgeis limited to appearances. His distinctive argument exhibits a pattern which can beformulated in general terms: If representation or concept S is a necessary condi-tion for the possibility of objective knowledge, and if this can be established apriori and only a priori, then S can be true only of appearances and not of thingsin themselves. But, Trendelenburg objects that this pattern of argument is falla-cious; for, he argues, the transcendental premise is compatible with thinking thatS is true not only of appearances but also of things in themselves.

Is this a fallacy, and if so, what is the fallacy? There is reason to think that it mayindeed be a fallacy. For the a priori claim that representation or concept S (e.g., therepresentation of space) is a necessary condition for the possibility of objectiveknowledge in fact appears to be ambiguous. It may mean that being a certainnecessary condition for the possibility of objective knowledge is what constitutes,or part of what constitutes, the identity of this representation or concept. Or it maymean that being a certain necessary condition for the possibility of objectiveknowledge is something that is true, even necessarily true, of this representation orconcept but without being part of its identity. If Kant’s inference is based on theformer, stronger claim, then clearly it is valid. But then one may wonder how hecould succeed in defending such a strong claim. If, however, his inference is basedon the latter, weaker claim, then it does indeed appear to be fallacious. For then itappears that representation or concept S can without contradiction be said to be

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true of things, but without this reference to things involving a reference to con-ditions for the possibility of knowledge.

Whether Kant is guilty of this fallacy in the aesthetic is another matter. Todefend Kant against Trendelenburg’s charge of fallacy, we would need either toshow that he is aware of the distinction between the stronger and the weakerversion of the transcendental claim, and that he is deliberately arguing for thestronger; in particular, that he is not sliding from the weaker to the stronger. Orelse we would need to show that, in spite of the sound of his language, what hewants to conclude in the aesthetic is not that the representations of space and timecannot be true of things in themselves but only that, since our argument for theobjective validity of these representations, that is, for the claim that they are trueof things, is transcendental, we cannot have reason to think that they are true ofthings in themselves, we can only have reason to think that they are true ofappearances. Eventually, of course, Kant does want to defend the claim that theserepresentations cannot be true of things in themselves and not only the claim thatwe cannot have reason to think that they are true of things in themselves. But wemay suppose that it is only in the Antinomies that he argues for the former,stronger claim, whereas the aesthetic argues only for the latter, weaker one.

Trendelenburg’s fallacy objection is today commonly understood as just that, acharge of fallacy. But for Trendelenburg this charge is only a first step in a positiveaccount of how knowledge is related to reality. He shares with Kant the transcen-dental premise, especially in regard of the representations of space and time, buthis aim is to show that these representations are at the same time true of things inthemselves. The fallacy objection is a crucial, but only a first step in defending thispositive account. It is important that we should recognize this positive and sys-tematic context of Trendelenburg’s objection; for there is evidently a big differ-ence between claiming that there is no logical contradiction in supposing thatrepresentation or concept S is both a transcendental condition and true of things inthemselves and claiming that S is both a transcendental condition and true ofthings in themselves. Trendelenburg defends both claims, and he defends theformer as a necessary first step to defending the latter.

It might be thought that even if Trendelenburg regarded the fallacy objection aspart of a positive account of the relation between knowledge and reality, theobjection can be assessed independently of this context. But this does not appearto be so. For suppose that the fallacy objection is sound. It is not clear that theKantian need be especially perturbed by it. Yes, she may grant, supposing thatrepresentation or concept S is a transcendental condition is logically compatiblewith supposing that S is true of things in themselves. But what positive groundscan we have for the latter supposition? The fact alone that its conjunction with thetranscendental premise does not involve a logical contradiction does not amountto any positive grounds. In other words, unless the fallacy objection is supple-

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mented with a defense of the positive claim that S is true of things in themselves,the Kantian can render it innocuous by formulating her argument more cau-tiously: If our grounds for thinking that a representation or concept S is true ofthings is that S is an a priori necessary condition for the possibility of ourknowledge of things, then we have conclusive grounds for thinking that S is trueof appearances, but we cannot have any grounds for thinking that it is true ofthings in themselves. Arguably, this more cautious version of the Kantian argu-ment and conclusion does not give up any fundamental Kantian commitments,and it is sufficient to establish the central Kantian claim that our knowledge islimited to appearances. Alternatively, if the Kantian is committed to the strongerclaim that S cannot be true of things in themselves, she/he may appeal to theantinomies for an argument.

Distinctive of Trendelenburg’s positive account of the relation between knowl-edge and reality is the claim that we must represent what we experience as beingin space and time because what we experience is, in itself and without referenceto us or our knowledge, in space and time. “Sind sie [the representations of spaceand time] vielleicht nicht gerade darum für den Geist notwendig, weil sie es fürdie Dinge sind?”4 Moreover, Trendelenburg combines this realist “because” withthe claim that the representations of space and time are transcendental conditions,that is, a priori necessary conditions for the possibility of objective knowledge.This combination of realism with the transcendental premise goes against the verycore of the Kantian project, for it denies that the acceptance of the transcendentalpremise must be associated with any kind of idealism and any Copernican turn. Itis hard to imagine a more radical objection to the Kantian project than the claimthat the transcendental premise does not require the claim that a transcendentalrepresentation or concept is true of things because it is true of our knowledge ofthings but is compatible with the claim that, on the contrary, a transcendentalrepresentation or concept is true of our knowledge of things because it is true ofthings.

So far, Trendelenburg’s objection is that the transcendental premise does notrequire an antirealist account of the explanatory priority between knowledge andreality; it is compatible with a realist account of this priority. But this allows himto argue, in a more positive vein, that the transcendental premise requires a realistaccount of this priority. His basic reasoning appears to be simply this: The very aimof the transcendental premise is to show how there can be objective knowledge, butknowledge is objective without qualification only if it is of how things are in

4 “Are they [the representations of space and time] not perhaps necessary for the mind for this veryreason, namely, because they are necessary for things?” A. Trendelenburg, Logische Untersuchun-gen, 3rd ed. (Leipzig: Hirzel, 1870) 163. The tone of the question is polemical: A positive answer isprecisely what Trendelenburg will defend.

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themselves. Of course, if things in themselves are in space and time, and if therepresentations of space and time are transcendental conditions, then things inthemselves satisfy these conditions for our knowing them. But it does not followthat we cannot know things in themselves but can know things only relative tocertain essentially epistemic conditions. For it is one thing to say that a represen-tation or concept S is true of things and it is a transcendental condition, and anotherto say that S is true of things because it is a transcendental condition. And S, evenif it is an epistemic condition, and necessarily so, need not be so essentially: Whatthis representation or concept is, need not make reference to epistemic conditions.

Trendelenburg’s reasoning, certainly, involves a certain conception of objectiv-ity, namely, that if knowledge cannot be of things in themselves, then it cannot beobjective without qualification. The emphasis here is very much on the withoutqualification, and it amounts to claiming that the Kantian limitation of knowledgeto appearances is a substantive restriction of objectivity and not a mere articulationof the concept of objective knowledge.

Trendelenburg’s overall argument, for sure, invites the Kantian objection that acausal account of the relation between knowledge and reality can ground onlyempirical knowledge, not a priori knowledge. Trendelenburg recognizes thisobjection, and his response is that it is based on an excessively empiricist accountof the causation involved in knowledge. His point is that the Kantian objectionsupposes that we cannot infer anything about the nature of the cause from thenature of the effect, but this supposition Trendelenburg denies. His argumentdepends on the possibility of inferring what the cause must be like on the basis ofwhat the effect is, essentially, like (see also below).

II.2. The Relevance of Trendelenburg’s Response to Kant forOur Understanding of Aristotle

It would be natural to say that Trendelenburg’s account of the relation betweenknowledge and reality combines Kantian and Aristotelian elements. The Kantianelement is the transcendental premise; the Aristotelian elements we shall identifypresently. However, this view of Trendelenburg’s relationship to Kant and Aris-totle would not, I think, properly appreciate the distinctiveness and force ofTrendelenburg’s response to Kant. For the point of Trendelenburg’s dissociatingthe transcendental premise from transcendental idealism is, apparently, to denythat there need be anything peculiarly Kantian about the transcendental premise,that is, a premise of the form: Representation S is a necessary condition for thepossibility of objective knowledge, and this can be established a priori and onlya priori.

What then, according to Trendelenburg, is the mistake behind the association ofthe transcendental premise with transcendental idealism? We may suggest that the

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mistake lies in confusing the transcendental premise, in the strict and sober sense,with such a premise when combined with the commitment to a certain method inmetaphysics and a certain view about the relation between epistemology andmetaphysics. For Trendelenburg’s point can be understood as being that it is onlyif we suppose that epistemological claims can be sharply separated from meta-physical ones, and that the latter must be based on the former, that the Kantianassociation of the transcendental premise with idealism can be at all justified. Byitself, the transcendental premise implies no such association.

I shall first indicate schematically a number of apparently Aristotelian elementsin Trendelenburg’s account of the relation between knowledge and reality, beforelooking more closely at the use to which they are put by Trendelenburg and askingwhether this use can contribute to our understanding of Aristotle.

First, there is what we may call the method behind Trendelenburg’s argumentfor transcendental realism. For although the main premise of this argument is thetranscendental premise, the method of the argument could not justly be describedas transcendental. This is because it directly rejects two distinctive features ofa transcendental method, certainly as this method is familiar from Kant andbecomes prominent in Cohen and Natorp. These features are, first, that epistemo-logical and metaphysical claims must be kept sharply separate, and second, thatthe latter must be grounded in the former. Trendelenburg’s method of argumentand inquiry, as he expressly characterizes it, is to start with an initial point ofinquiry in which we know the Erkenntnisgrund of what we are searching for, butnot its Realgrund, and to work toward a final point of inquiry in which we knowboth the Erkenntnisgrund and the Realgrund. At this final point, the Realgrundexplains the original Erkenntnisgrund and, therefore, constitutes a final and com-plete Erkenntnisgrund. But this method of argument and inquiry appears to bemodeled directly on Aristotle’s method (e.g., in the Physics) of starting with whatis better known to us and searching for its explanation in what is more intelligiblein nature and essentially. We may note that this Aristotelian element was imme-diately recognized by Cohen.5

Second, there is Trendelenburg’s conception of causation in general, whichallows him to argue that we can (at least in certain cases) infer the nature of thecause from the nature of the effect—the antiempiricist conception. In Aristotle,such a conception is found in his account of efficient causation as form-based, thatis, the claim that a moving cause is causally efficient in virtue of its form; hence,that what is caused must, in some way, share the form of what causes it. Of course,the view that there is a necessary and essential connection between cause andeffect, and a relation such that we can infer the nature of the cause fromthat of the effect, need not be distinctively Aristotelian. For one thing, this view

5 See Cohen (1871): 33f.

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appears to be shared by Plato (in the Phaedo), and it may be said to be a viewdistinctive of rationalism generally. But the use to which it is put by Trendelen-burg, that is, to provide a causal realist account of knowledge, and in particular ofknowledge based in sense-perception, does point to Aristotle.

Third, there is Trendelenburg’s peculiarly causal account of empirical repre-sentation, or sense-perception. For he argues that empirical representation, inorder to represent physical motion (Bewegung)—and hence represent things asbeing in space and time—must (1) itself be a motion of some sort, and (2) becaused by things that are in physical motion and insofar as they are in physicalmotion. This looks very much like the De Anima account of sense-perception,which says that to perceive F is to be causally affected by a thing that is F in sucha way that its being F causes the sense faculty to be F in some way. Of course,Aristotle carefully addresses the way in which it can be said that the sense facultybecomes F in perceiving a thing that is F, and he provides an account of how thiscausal process is to be understood, namely as the reception of the form without thematter. Trendelenburg does not go into such detail, and this does make his theoryof sense-perception appear underdeveloped—unless, that is, we see it as tacitlypresupposing considerable familiarity with the Aristotelian theory.

Fourth, Trendelenburg appeals to this peculiarly causal account of sense-perception for the purpose of defending an unqualifiedly realist account of knowl-edge: knowledge of how things are, in themselves and without qualification. Thepoint is not simply that Aristotle’s causal theory of sense-perception can beappropriately described as realist—though this is no doubt true—but that thisrealist use is one to which the causal theory of sense-perception is, consciouslyand deliberately, put by Aristotle. He does this when he argues against certainattempts, by philosophers which he describes as relativists and phenomenalists, toconfine perception to phenomena and deny that it can be of things, things inthemselves and without qualification (Metaphysics Gamma 5; see below).

Suppose, then, that these are indeed Aristotelian elements. We ought still to askwhether the distinctive use to which they are put by Trendelenburg is remotelyAristotelian. For Trendelenburg’s aim, we have seen, is not only to defend a causalrealist account of unqualifiedly objective knowledge, but to do so as part ofaccepting and seeking to defend the transcendental premise which says that therepresentations of space and time are conditions for the possibility of objectiveknowledge, and that this can be established a priori and only a priori. Thiscombination of causal realism and the transcendental premise, we might suppose,cannot possibly be Aristotelian, in origin or inspiration; rather, it amounts totaking a distinctively Kantian element, divorcing it from the Kantian context, andtransplanting it into a very different context. Perhaps so. But we ought not to ruleout in advance that Trendelenburg may prompt us to reconsider our understandingof Aristotle even to the extent of our being ready to try out what might be called

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a transcendental realist interpretation of Aristotle’s account of the relationbetween logic and metaphysics, that is, between the principles of thought and theprinciples of reality. Certainly Trendelenburg has justly prompted us to reconsiderany assumption that a transcendental premise must be associated with some kindof antirealism, and this is sufficient to ensure that a transcendental realist inter-pretation of Aristotle need not be a misplaced anachronism.

I think a transcendental realist interpretation of Aristotle is worth consideringand taking seriously—and this is a particular benefit of reading Aristotle againstthe background of Trendelenburg. It is not perhaps commonly recognized that thequestion of the relation between logic and metaphysics is expressly raised byAristotle: in the second aporia in the third book of the Metaphysics (996b26–997a15), which asks whether it is the task of a single science to investigate boththe ultimate principles of being and the basic principles of reasoning or this is thetask of fundamentally different sciences. This question is then taken up, andanswered, in the fourth book of the Metaphysics (esp. chs. 3–6), where he arguesthat the principle of non-contradiction (PNC) which is paradigm of a logicalprinciple, is at the same time a metaphysical principle and, further, that it is logicalprinciple precisely because it is a metaphysical principle.

As is familiar, Aristotle argues that we must conform to PNC in our thinking ifwe are at all to think intelligibly about anything. In this sense, PNC is a logicalprinciple. Aristotle asks us to consider someone who denies that PNC is true ofthings, and he sets out to derive the consequence that the thinking of such a personflouts an evident and minimal requirement of thinking about something, namely,that to think about something is to think about one thing rather than another—it isin this sense to think about something determinate (horismenon). It is notable, forour purposes, that no premise in this argument could appropriately be described asempirical. In this sense, there is in Aristotle what we may call a central transcen-dental claim: The claim says that the supposition that PNC is true of things is anecessary condition for object-directed thought, and this can be established apriori and (presumably) only a priori.

This, however, is only the first part of Aristotle’s defense of PNC: It defendsPNC as a logical principle. Aristotle recognizes that a great deal more is neededto defend PNC also as a metaphysical principle: a principle not only aboutthought, which is true of things insofar as they are thinkable, but about things,which is true of things without qualification and simply insofar as they are real.What is additionally needed, he thinks, is addressing and answering the challengeof those who draw a sharp distinction between phenomena and the things them-selves, and who appeal to this distinction to argue that even if Aristotle has shownthat we must suppose that PNC is true of phenomena, that is, things insofar as theyare perceivable and conceivable by us, he has not shown that we must suppose thatPNC is true of the things themselves and without qualification.

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The first thing to note is that it is Aristotle that raises this challenge, on behalfof the so-called phenomenalists and relativists, and that he appears to take itentirely seriously. The second is that he invokes what looks like a causal realistaccount of sense-perception, and in general of our knowledge of things, preciselyto answer this challenge. The challenge was that even if things must appeardeterminate to us, and hence subject to PNC, if it is to be possible for us to thinkof them, it does not follow that the things, themselves and without qualification,must be correspondingly determinate and hence subject to PNC. Aristotle’sresponse to this challenge is to argue that it is because the things, themselves andwithout qualification, are determinate that they appear determinate to us (Meta-physics Gamma 5, 1010b30–1011a2).

In the context of contemporary Aristotle scholarship, the suggestion that Aris-totle defends what we may appropriately describe as a transcendental causalrealist account of the relation between logic and metaphysics, or the principles ofthought and the principles of reality, ought to be of some interest. First, it issometimes thought that Aristotle does not raise the general question of the relationbetween thought and reality, or certainly that he does not raise it in a radicalway—as Kant does—that is, by recognizing and taking seriously the possibilitythat the two may come apart. But this view of Aristotle is arguably mistaken. Hedoes expressly raise this question (in book 3 of the Metaphysics). Moreover, thefact that, in setting out to answer it, he comes round to considering the challengethat a fundamental logical principle such as PNC may be true only of appearancesand not of the things themselves and without qualification, and that apparently hetakes this challenge seriously and answers it directly, shows that he raises thequestion of the relation between thought and reality in a thoroughly radical way.

Second, we may recall a relatively recent debate between Martha Nussbaumand John Cooper.6 Nussbaum argued that the conclusions of Aristotle’s dialecticalarguments (arguments based on endoxa, “reputable opinions”) aim not at unquali-fiedly objective knowledge but at knowledge relative to our point of view. JohnCooper replied that the very distinction between unqualifiedly objective knowl-edge and knowledge relative to our point of view (we may compare the distinctionbetween internal realism and external realism) is an unjustifiable projection of aKantian and post-Kantian distinction onto a philosopher who does not contem-plate the possibility of a radical dissociation between appearance and reality. Butwe do not have to choose between these two opposed interpretations. AgainstNussbaum, we may maintain that Aristotle’s dialectical arguments can and do aim

6 M. C. Nussbaum, “Saving Aristotle’s Appearances,” Language and Logos, ed. M. Schofield andM. C. Nussbaum (Cambridge, UK: Cambridge UP, 1982) 267–93; J. M. Cooper, “Aristotle on theAuthority of ‘Appearances,’ ” reprinted in his Reason and Emotion (Princeton, NJ: Princeton UP,1999) 281–91.

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at unqualifiedly objective knowledge; against Cooper, that Aristotle does contem-plate the possibility of a radical dissociation between appearance and reality, buthe wants to resist it and he argues directly against it.

Third, Aristotle’s philosophy is today often described as naturalist. But is this afitting description of his account of the relation between thought and reality?Naturalism, it is true, is naturally associated with a causal realist account of therelation between thought and reality. Does it follow that Aristotle, since he defendssuch an account, defends a naturalist account of this relation? One reason to thinkthat it may not follow is that Aristotle—at least if the Trendelenburgian interpre-tation has been on the right lines—combines a causal realist account of therelation between thought and reality with a transcendental account, and thiscombination will not sit easily with naturalism, certainly not as naturalism iscommonly understood.

III. NATORP AND PLATO

III.1. Natorp’s Claim That Forms Are Explanations, Not Substances

Natorp’s interpretation of Plato’s theory of forms contains two major claims, apurely metaphysical and a transcendental idealist claim. He is not always carefulto observe a distinction between them and he tends to run them together, but it willbe useful to keep the two claims distinct and consider them separately:

1. Plato’s forms are laws—in the sense of objective, real explanations(aitiai, i.e., explanantia)—not substances.

2. Plato’s forms are primarily elements in the nature of thought andknowledge, and only as a consequence are they elements in the natureof reality.

The second claim is central, for it summarizes Natorp’s transcendental idealistinterpretation of Plato. But the first is a purely metaphysical claim, which canstand on its own, and this means that we may find the conception of Plato’s formsas explanations, not substances, attractive independently of the transcendentalidealist conception of the forms.

“The ideas signify laws, nothing else” (p. 49).7 This claim may strike one asimporting into Plato, in a way that is clearly anachronistic, a modern conceptionof laws of nature. But this impression would be to misunderstand Natorp. For by

7 References are to Platos Ideenlehre, as reprinted in Meiner Verlag. The translations are by JohnConnolly and me, taken from Paul Natorp, Plato’s Theory of Ideas. An Introduction to Idealism. Editedwith an introduction by Vasilis Politis. Translation by Vasilis Politis and John Connolly. Postscript byAndré Laks. International Plato Studies, Vol. 18 (Sankt Augustin: Academia Verlag, 2004).

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a law he means, basically, a general explanation of why a plurality of cases are asthey are. Here explanation refers not so much to our explanatory theories, but toobjective, real explanations, which we search for in our explanatory theories. Andit is arguable that this notion of an explanation is present in Plato. The locusclassicus is Phaedo (95e ff.), where Plato argues for the striking claim thatexplanantia (aitiai) are forms or essences, or rather, that explanantia, whateverelse they may involve, are primarily forms or essences. The argument for thisdistinctive claim is basically that only if we suppose that the claim is true can wedefend certain central requirements of explanation and in particular the require-ment that explanations must be uniform: same explanandum if, and only if, sameexplanans. Properly spelled out, the requirement of uniformity directly impliesthat distinct particulars, insofar as they have the same qualities, must have thesame explanations. And this does sufficiently indicate that Plato’s aitiai are indeedgeneral explanantia of why a plurality of cases are as they are.

Even if we agree that Plato thinks that forms are general explanantia, what isdistinctive of Natorp’s claim is that the forms are explanantia and not substances.On a common and traditional interpretation of the Phaedo claim,

(Interpretation 1) The forms are a kind of substance, and this kind of substance is capable ofexplaining other things.

But what Natorp defends is the very different interpretation:

(Interpretation 2) The forms are explanantia, not substances.

What exactly is Natorp denying when he denies that forms are substances? He isnot, of course, denying that forms are primary beings, on the contrary, theirontological primacy is a central part of what he is trying to account for. Certainlyhe is denying that the nature of forms can be understood on the analogy of theobjects with which we are directly familiar in experience—except that whereasthe latter are changing, forms are supposed to be changeless. Stated more care-fully, he is denying that to be a form is to be a primary subject of predication;denying the view that forms are the primary subjects of predication and that theobjects with which we are directly familiar with in experience are subjects ofpredication only insofar as they are appropriately related to forms. The notion ofsubstance that Natorp is operating with, when he denies that forms are substances,is the Aristotelian notion of an ultimate subject of predication.

Our question is whether this remarkable claim, that forms are explanantia andnot substances, can contribute to our understanding of Plato’s theory of forms.There are two questions here. First, what difference does it make to our overallunderstanding of Plato’s theory of forms whether we adopt the traditional inter-

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pretation 1 or Natorp’s interpretation 2? Second, do we have any good reason tothink that Plato intends interpretation 2 and not interpretation 1? I shall begin withthe first question.

Natorp claims a number of major benefits of this interpretation compared to itstraditional counterpart, benefits that it will be useful to trace. He claims fourbenefits. First, interpretation 2, unlike 1, allows us adequately to understand whatthe ontological primacy of the forms consists in. Second, it allows us to realizethat the forms are essentially related to nature, thus avoiding the danger of adissociation between a world of forms and a world of sensible phenomena—thedanger that Plato alerts us to in the first part of the Parmenides. Third, it allows usto realize that the theory of forms is not subject to a third-large or third-manregress—the threat of a regress that Plato articulates in the Parmenides and thatapparently Aristotle thought decisively undermines the theory. Finally, it providesan account of the relation between forms and sensibles, the relation that Platorefers to as “participation” but otherwise appears to leave unexplained. Let us lookmore closely at each of these supposed benefits.

III.1.1. The Ontological Primacy of the Forms

On Natorp’s interpretation it follows directly that Plato’s forms are not tran-scendent substances, for they are not substances at all. So they are not inhabitantsof a world of changeless substances beyond this spatiotemporal world of changingones. In this way Natorp wants, at a stroke, to rescue Plato’s theory from acriticism first leveled against it by Aristotle and one that it has been difficult toshake off ever since. The criticism is that Plato’s forms are nothing but changelessand everlasting duplications of the familiar world of changing and perishablethings (Metaphysics III. 2, 997b5–12; see also XIII. 4, 1078b34–36). However,he emphasizes that Plato’s forms, which he understands to be explanantia, areindeed, in a different sense, separate. For they are independent of what theyexplain, namely, changing things, and changing things depend, for their existenceand nature, on the forms that explain them—changing things ontologicallydepend, in this sense, on changeless things. Plato, he argues, is committed to thegeneral principle that what explains is primary and independent—and, in thissense, “separate”—whereas what it explains is derivative and dependent.

Natorp argues that the conception of the forms as explanantia fits perfectlyPlato’s view that the forms are ontologically primary. For, he argues, things areontologically dependent on their explanations—things are, he does not tire ofsaying, constituted by their explanations: “It is, quite generally, the law, [. . .] theeidos or idea, that constitutes objects (to on)” (p. 50). It follows that abstract,general laws are primary whereas concrete particulars are ontologically dependenton such laws. This is evidently a controversial view. Directly opposed is the view

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that causes and effects, or explanantia and explananda, are distinct and indepen-dent entities. But Natorp argues that the individual case explained by a law isontologically dependent on the law that explains it, and he ascribes this view toPlato.

Is it right to attribute to Plato the view that things are ontologically dependenton their explanations? The Phaedo passage (95e ff.), whether it is read accordingto interpretation 2 or according to the traditional interpretation 1, supports thisview. For Plato argues, on the one hand, that the forms are causes or explanantiaof things, and in particular of the qualities of things. For example, the form of thebeautiful explains why any thing that is beautiful is beautiful. But he also argues,on the other hand, that the form of a quality, for example, the quality of beingbeautiful, is what constitutes the identity of this quality. For example, the form ofthe beautiful is what it is to be beautiful (see e.g., Phaedo 101c2–5). In general,whatever precisely Plato’s forms are, certainly they are what is defined in Socraticdefinitions, that is, what is known when the answer to a Socratic question isknown, for example, the question “What is it to be beautiful?” Plato’s forms,whatever else is true of them, are essences. It follows, crucially, that: That whichexplains why a particular thing has a certain quality, F, and that which determineswhich quality this is and constitutes the identity of this quality, are one and thesame thing. But if we put together these two sides of Plato’s theory of forms, thatis, forms as explanantia and forms as essences, it follows that one and the samequality, F, insofar as it is had by a particular sense-perceptible and changingparticular, could not, on pain of not being the very quality it is, have a differentcause or explanans from the one that it actually has. Plato, then, argues thatthings—at least to the extent that they are constituted by their qualities—areontologically dependent on their causes or explanantia.

If, however, the ontological primacy of the forms follows on either interpreta-tion 1 or 2, why does Natorp claim a particular benefit for interpretation 2 in thisregard? Interpretation 1 can indeed account for why the forms are prior to thethings they explain and in particular sensible things. But can it account for whythey are primary without qualification? And is interpretation 2 in a better positionto do so? These are not easy questions, but here is an attempt to identify theplausibility behind Natorp’s answer. Suppose (in accordance with interpretation1) that it is one thing for a form to be, namely, for a form to be is for it to be achangeless substance, and it is quite another for a form to be an explanans. True,in virtue of its being an explanans, it will, according to the Phaedo claim thatexplanantia are primarily essences, be ontologically prior to what it explains. Butit will be primary without qualification not in virtue of being an explanans, but invirtue of being a changeless substance. This shows that, on interpretation 1, theexplanatory primacy of the forms will at best explain their relative ontologicalpriority, not their absolute ontological primacy. And the question will still remain,

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why forms, insofar as they are changeless substances, should enjoy relative andindeed absolute ontological primacy. If, on the other hand, we adopt interpretation2, the explanatory primacy of the forms will account not only for their relativeontological primacy, but also for their absolute ontological primacy.

Natorp, we ought to note, is not guilty of having Plato simply identify the notionof ontological dependence with that of explanatory dependence. These are clearlydifferent notions: X depends on Y for its being the very thing it is (ontologicaldependence); X depends on Y for its being F, or for its existing (explanatorydependence). The former notion needs the notion of essence (“being the very thingit is”) for its articulation, the latter does not. What Natorp is doing, as I understandhim, is arguing that, according to Plato, ontological dependence is derivable fromexplanatory dependence. The derivation is based, first and foremost, on the claim,which Plato states and defends in the Phaedo (95e ff.), that: That which explainswhy a particular thing has a certain quality, F, and that which determines whichquality this is and constitutes the identity of this quality, are one and the samething.

III.1.2. The Other-Directedness of the Forms and Their Relation to Nature

If Natorp’s interpretation is correct and the forms are explanantia and notsubstances, then Plato’s forms are, by their very nature, other-directed. For anexplanans is, essentially, an explanans of something and of something other thanitself. Perhaps explanantia can also explain themselves, but this cannot be all thatthey explain. Thus, the form of the beautiful is the explanans of something, notsimply of the form of the beautiful, but of beautiful things—that is, of anythinginsofar as it is beautiful or of why anything that is beautiful is beautiful. Indeed,the Phaedo makes clear that what the forms ultimately explain is changing things,things in nature. For in the same passage (Phaedo 95e ff.), Plato makes a point ofrepeating that what, for example, the form of the large explains is why somethingis or comes to be or ceases to be large, and this shows that the explanandum is hereunderstood as something changing, indeed changing in respect of largeness.

Natorp does not tire of emphasizing this other-directedness of the forms: “[A]sfunctions and not things, they [the forms] do not dispense with directedness, butare in fact entirely dependent on it” (p. 276). Of course, even if the forms areexplanantia and not substances, they will still be things, entities, for they are afterall supposed to exist. But Natorp is arguing against the view that forms are theprimary subjects of predication and that the objects with which we are directlyfamiliar with in experience are subjects of predication only insofar as they areappropriately related to forms. One important thrust in his argument is, precisely,that the notion of a thing or substance, in the sense of an ultimate or primarysubject of predication, unlike the notion of an explanans, is not the notion of

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something other-directed. For example, to be a stone is not to be a stone ofsomething else or in relation to something else. We may also note that even if thestone is changeless and everlasting, even if, for example, it is like a heavenly body(on a certain conception of heavenly bodies), still this does not make it other-directed. So even if the forms are transcendent substances, that does not makethem other-directed. Indeed, even if the stone is capable of causing and explainingthings, for example, if it is capable of explaining why the window broke, still thisdoes not make it something other-directed. For, if the stone is a substance, it is notpart of its essence that it contributes to particular causal explanations. So, on thetraditional interpretation of the forms as changeless substances (i.e., interpretation1), the forms are not other-directed, at least not essentially. On the other hand, ifforms are, purely and simply, explanantia, that is, if this is part of their essence(i.e., interpretation 2), then they can be characterized only by reference to whatthey explain, so they will be essentially other-directed.

So if the forms are explanantia and not substances, they will be essentiallyother-directed. But what they are directed at is, in particular, changing things. Forit is, ultimately, changing things that the forms are supposed to explain. It followsthat the forms are, by their very essence, directed at changing things and at nature.Natorp concludes that the forms are ontologically primary but at the same timehave an essential reference to nature.

It may be objected against Natorp’s view of the other-directedness of the formsthat, contrary to what Plato thinks, it makes the forms dependent on the existenceof changing things, since changing things is what they are ultimately supposed tobe directed at and explain. This objection, however, is I think based on a misun-derstanding of his view. As I understand it, Natorp’s claim that forms are essen-tially other-directed is not supposed to entail that there exists something at whichthey are directed. What it means is, rather, that the account of what it is to be a formwill contain ineliminable reference to changing things—changing things in generaland as a kind rather than any particular changing thing. Just as the account ofwhat it is to be an explanation contains ineliminable reference to explananda—explananda in general and as a kind rather than any particular explanandum. If itis objected that this still makes the forms dependent on nature, it may be respondedthat this is a dependence between the form of forms and the form of nature and thatsuch dependence may well be in conformity with what Plato thinks.

III.1.3. The Response to the Third-Man Argument

In the Parmenides (132a) Plato puts forward a general argument against his owntheory of forms which, if successful, directly undermines the theory. The argumentruns as follows. The set of sense-perceptible things that are, for example, large arelarge because of the form the large, and this form explains why each of these things

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is large, but now consider the set of things that includes both the sense-perceptiblethings that are large and the form of the large; it may seem that a further form isneeded to explain why the things in this set are all large, but in that case, there willbe not one form of the large, but infinitely many, and this is absurd, since theexplanation of why things are large can never come to an end—or indeed even getproperly started. This argument, which Aristotle refers to as the third-man argu-ment and raises against Plato, may seem powerful. At the same time, it is doubtfulwhether Plato thinks the argument is at all successful. For, having once raised it, henever explicitly answers it, and he apparently continues to state the theory of formsin much the same terms in dialogues that, on most views, are later than theParmenides (see e.g., Timaeus 27–28, and the end of the Philebus).

Natorp argues that the so-called third-man argument never posed a serious threatto the theory of forms. For the conception of the forms as laws and explanantia, notsubstances, provides, he argues, a direct and conclusive response to it:

The argument only succeeds if it is assumed as obvious that horseness, i.e. the conceived totality ofmarks that define what a horse is, is present only in horses themselves, but that Plato distinguishesbeing-a-horse from particular horses and thinks of being-a-horse as some further thing—literally asecond horse. In that case, of course, there is no reason why this should not continue ad infinitum.If, however, one thinks, as Plato has been seen to do, only of the relation between a particular caseand a law, [. . .] then it may of course be true that the law in question relies on a further and morefundamental law, hence there is indeed nothing to prevent that there should be an idea of an idea;but there is no longer any danger that this should proceed to infinity. (p. 412)

If the forms are explanantia, they are not at all like the things that they explain;certainly the difference between them is not simply that between changing andchangeless things. But then we cannot add the form of the large to the set ofsense-perceptible things that are large and ask with regard to this expanded setwhat explains the largeness of each of its members; as we can, apparently, if thedifference between sense-perceptible things and the forms is simply that betweenchanging and changeless things. For we cannot ask for the explanation of theexplanans, that is, of something whose very essence is to be an explanans, at leastnot in the same way as we can ask for the explanation of the explanandum. Sothere is no danger of there being infinitely many forms of the large, rather, thereis one explanans, hence one form, of the large. But there may still be forms ofparticular features of the form of the large, for example, why, like every form, theform of the large is one, perfectly unitary, changeless, etc.

III.1.4. The Account of Participation

Natorp’s response to the third-man argument, and in general his conception ofthe forms as explanantia and not substances, is associated with a striking response

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to what has, ever since Aristotle’s criticism, been seen as a central problem forPlato’s theory: What is the relation between concrete, changing particulars and theforms—the relation that Plato variously refers to as “participation” and “com-munion” (methexis, koinonia) but otherwise appears to leave unanalyzed?Natorp’s response is that this is precisely the relation between a particular caseand the general law or explanation that explains it: “ ‘Participation’, in whichAristotle saw only a metaphor without exact significance, accordingly means quitesimply the relationship of a case to a law” (p. 155).

The reason why Plato did not further analyze this relation, he argues, is that hethought it is basic and unanalyzable. Certainly, as Plato argues in the Parmenides(132d–e), it must not be analyzed in terms of the relation of similarity. For thena regress of forms will again arise. But to say that the relation of participation isunanalyzable, because it is simply the relation between a particular case andits explanans, is not to say that we cannot clarify and help determine what thisrelation is. For we can give an account of what the relation of explanation is.For example, we may point out that this relation is essentially asymmetrical(i.e., if X explains Y, then Y does not explain X), as Plato did in the Euthyphro(pp. 9–11). Or we may point out certain general requirements essential toexplanation, such as the following: same explanantia (i.e., what explains) havesame explananda (i.e., what is explained) and same explananda have sameexplanantia. Plato in fact defends these requirements of explanation in thePhaedo (95e ff.).

III.2. How Credible Is Natorp’s Claim That Forms AreExplanantia, Not Substances?

We have seen that the choice between the two interpretations of Plato’s claimthat forms are explanantia (aitiai) makes considerable and arguably fundamentaldifference to our overall understanding of the theory of forms. But is it a genuinechoice? Do we have any good reason to adopt interpretation 2 in preference tointerpretation 1? One thing to observe is that critics have not typically raised thequestion of which of the two interpretations we should adopt; typically they haveassumed that Plato intends interpretation 1, without recognizing the possibility ofan alternative interpretation. But it seems highly important to raise this question,whichever way we eventually answer it.

How, then, can we go about trying to decide between interpretations 1 and 2? Orshould we perhaps rather suppose that Plato may not have recognized the differ-ence and that it may, therefore, not be possible to argue for a decision either way?I think a case, based on a comprehensive reading of the Phaedo, can be made infavor of interpretation 2. Here I can only begin to sketch the case, the centralpremise of which I have defended elsewhere. The central premise is that:

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Plato’s argument, in the Phaedo, for the claim that explanantia are, primarily, forms or essences issufficient to introduce the concepts of form and essences and to defend the claim that there areforms and essences.

What this premise denies is that the argument in the Phaedo, for the claim thatexplanantia are, primarily, essences, depends on a prior and independent conceptof form/essence and a prior and independent commitment to the existence offorms/essences.8 I hasten to add that this central premise is not supposed to implythat Plato arrived at the concept of form/essence, or the commitment to theexistence of forms/essences, through the search for an adequate account of expla-nation. Arguably, he had independent ways of arriving at this concept and com-mitment. What the central premise implies is, rather, that, to the extent that thetheory of forms/essences is associated with a theory of explanation, it can bedefended through the attempt to develop and defend an adequate theory ofexplanation—where this attempt does not presuppose a prior commitment to theexistence of forms/essences or even the concept of form/essence.

The relevance of this central premise ought to be immediately evident. For itdirectly implies that, to the extent that Plato’s theory of forms is defended throughthe attempt to develop and defend a theory of explanation, we have reason tosuppose that forms/essences are, essentially, explanantia. Of course, to say thatforms/essences are, essentially, explanantia is not at all to say that the concept ofessence/form is the same as the concept of explanation. Certainly it is not at allobvious that forms/essences are explanantia. On the contrary, the two concepts areclearly distinct: The latter concept is associated with the question, “Why is a thingthat is F, F?”; the former concept is associated with the question, “What is it fora thing, F, to be this thing F?” And there is not immediate reason to think that wecan answer the Why question only by answering the What question. What theclaim means, which says that forms/essences are, essentially, explanantia is that(according to Plato) if we reflect on what is required adequately to answer the Whyquestion, then we can see that this requires, first, introducing the concept ofform/essence, second, supposing that there are forms/essences, and third, con-cluding that any answer to the Why question will be of the form: X is F becauseX is appropriately related to the essence of the quality F, Ess(F). But this is not theplace to defend the “central premise.”

8 I cannot here properly address the difficult question of the relation between the notion of form andthe notion of essence in the Phaedo or whether the argument for the claim that explanantia areforms/essences depends strictly on the notion of form or more generally on the notion of essence. Ihave addressed these questions in “Explanation and Essence in Plato’s Phaedo,” Definition inAncient Philosophy, ed. D. Charles (Oxford, UK: Oxford UP, 2008, forthcoming).

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III.3. Natorp’s Transcendental Idealist Interpretation of the Theory of Forms

Natorp’s most distinctive thesis is that:

Plato’s forms, or at any rate the most fundamental and general ones, determine primarily the natureof thought and knowledge, and only as a consequence do they determine the nature of reality—thereality which is subject to thought and knowledge.

The Kantian affinity of this thesis is obvious. However, it is not as obtrusive as onemight fear. For Natorp takes issue with Kant on two crucial points. First, when hespeaks of the nature of thought and knowledge, he does not, unlike Kant, meansomething subjective, but rather something more fundamental than even thedistinction between subject and object: something that explains how subjects canthink and know and how objects can be thought of and known. Second, when hespeaks of reality, whose nature, he argues, Plato wants to derive from the nature ofthought and knowledge, he means reality itself, and not, unlike Kant, realitymerely as it appears to thinking and knowing subjects. So Natorp argues that itwas Plato that originally fashioned the transcendental idealist project, that is, theproject of deriving the nature of reality from the nature of thought and knowledge,but that this original transcendental idealist project is not, as it appears to be inKant, associated with any subjectivist connotations.

Is not the transcendental idealist thesis absurd as an interpretation of Plato? Forit seems to make the forms dependent on something subjective, namely, thethoughts and cognitions of thinkers and knowers. But, we may object, Platoexpressly rejects the suggestion that the forms may be something subjective ordependent on something subjective when he argues that the forms are not thoughts(noemata) or merely in the mind (psuche).9 This objection, however, is based ona misunderstanding. For when Natorp argues that Plato’s forms determine thenature of thought, he means that they are what accounts for the nature of object-directed thought and, as a consequence, of the objects of thought. He does notmean that the forms are thoughts or dependent on thoughts, if by “thoughts” wemean, as does Plato in the above passage from the Parmenides, individual acts ofthinking by individual thinkers.

Natorp’s transcendental idealist thesis, even when it is understood correctly, isof course highly contentious. Even if we grant that Plato’s forms are essentiallyrelated to thought and knowledge, and even if we grant further that they are

9 Parmenides 132b–c. For Plato’s express rejection of subjective idealism, see Myles Burnyeat,“Idealism and Greek Philosophy: What Descartes Saw and Berkeley Missed,” Philosophical ReviewXCI (1982): 3–40. But Burnyeat goes too far when he infers that not only subjective idealism, butany kind of idealism, is absent from Plato.

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introduced to make thought and knowledge possible, it may be going a step too farto conclude that the forms are primarily elements in the nature of thought andknowledge and only as a consequence elements in the nature of reality. We maystill object that since Plato’s forms are after all the primary beings, it follows thatthey are simply what primarily exists. But then we may conclude that they areprimarily elements in the nature of reality.

Natorp is acutely aware of this objection, and his response is this: If weunderstand the forms simply as that which primarily exists, we fail to ensure theiressential relation to thought and knowledge. For to think of something as beingwhat primarily exists is not necessarily to think of it as essentially related tothought and knowledge. Natorp concludes that to ensure that the forms areessentially related to thought and knowledge, we may indeed say that they areprimary beings, but we must add that they are primary beings precisely becausethey are elements in the nature of thought and knowledge. As he says, the formsare “not so much objective, as objectivising (positing objects)” (p. 239), that is, notso much objects, substances, as what accounts for the possibility of objects, theobjects that are subject to thought and knowledge.

Incidentally, it is worth remarking that if there is an insight in Natorp’s tran-scendental idealist thesis, and if the insight is that Plato’s forms must not beunderstood simply as that which primarily exists, this has also been recognized bysome recent critics who otherwise arrive at Plato from a very different angle. AlanCode says of the forms: “Plato’s realm of separable Being is not the realm ofexistence, though of course its inhabitants are supposed to exist. Rather, it is thedomain of definable entities—the objects about which one asks the Socratic“What Is X?” question—which is the question that Plato thinks is at the root ofknowledge.”10 Natorp argues that the transcendental idealist interpretation recom-mends itself naturally, if we understand correctly the relation between the theoryof forms, which is developed initially in such dialogues as the Phaedo and theRepublic, and the Socratic method of searching for definitions, which we encoun-ter from the earliest dialogues such as the Laches and the Euthyphro. For, heargues, it is a central aim of the Socratic method of searching for definitions toprovide an account of rational thought and speech and to do so purely from withinthe resources of such thought and speech. Therefore, if the Socratic method ofsearching for definitions is inseparable from the theory of forms, and if the theoryof forms grows naturally and inevitably out of the Socratic method, this likewisewill be the aim of the theory of forms: to provide an account of rational thoughtand to do so purely from within the resources of such thought. The theory of forms

10 Alan Code, “Aristotle: Essence and Accident,” Philosophical Grounds of Rationality, ed. Richard E.Grandy and Richard Warner (Oxford: Oxford UP, 1986) 426, original emphasis.

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thus emerges as a theory of rational thought, based in the resources alone of suchthought. Certainly, it is also a theory of reality, but only, Natorp concludes,because Plato wants to derive the nature of reality from the nature of rationalthought.

The aim of the Socratic method of searching for definitions, if we concentrateon its methodological side, is, Natorp argues, to develop an account of rationalthought and speech and to do so purely from within the resources of such thoughtand speech:

But one soon senses in these Platonic narratives that although the narrator is very much interestedin the content of these questions, i.e. in problems of morality and principles of action, he is alwaysequally or even primarily concerned with their formal aspect: with the general requirements of anadequate determination of concepts and with the laws of logical discussion and adequate argument.In these discourses we are witnessing the discovery and indeed the birth of the concept of logic.(p. 3)

Natorp identifies two main principles as those guiding the Socratic method ofsearching for definitions (see e.g., p. 4). First, rational thought and speech essen-tially involves having the same thought or making the same predication of manydifferent things (“this thing is F,” “that thing is F,” etc.). So, if one engages inrational thought and speech, one must be able to determine, or to search for thedetermination of, the identity of the single thing, concept or predicate that one isready to predicate of many different things. The search for an answer to theSocratic question, “What is this thing, the F?” is, he argues, precisely the searchfor “the unity, the identity of conceptual content” (p. 17). But Natorp’s point is notthat we must already know the unity and identity of conceptual content in order toengage in rational thought and speech. His point, rather, is that we must becommitted to there being such a unity and identity and be prepared to search forit. Second, rational thought and speech essentially involves rational, or logical,interrelations among different predications or concepts (“this is F, so it is not G,”“this is F only if it is G,” etc.). So, if one engages in rational thought and speech,one must be able to determine, or to search for the determination of, logicalinterrelations among different concepts. The Socratic elenchus, or method ofrefutation, which consists in searching for contradictions among beliefs or propo-sitions, is, together with the search for definitions, directed at this determination oflogical interrelations among predications and concepts.

It is interesting to compare Donald Davidson’s interpretation of the role in Platoof the Socratic method of the elenchus.11 Davidson argues that Plato never gave upthis method, which he first introduced in the early dialogues, of searching for

11 Donald Davidson, “Plato’s Philosopher,” Apeiron 26 (1993): 179–94.

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knowledge, and that he came to think, especially in the late dialogue, Philebus,that this is, ultimately, the only method of searching for knowledge, “in ethics atleast, and perhaps metaphysics more generally” (p. 187). But, Davidson asks, howcan Plato have thought that this method, whose immediate aim is consistency andcoherence within our beliefs, can lead to knowledge? He concludes that Plato’sview, that is, the view that the search for consistency and coherence in our beliefsis indeed a search for knowledge, is based on his “methodological assumption[. . .] that there is enough ineradicable truth in each of us to ensure that theelimination of inconsistencies ultimately results in the elimination of error” (p.186). So, according to Davidson’s interpretation of Plato, the search for the natureof the good life (ethics) and of reality (metaphysics) is to be conducted, funda-mentally, wholly from within our beliefs. The affinity with Natorp is striking,except that Natorp goes a decisive step further and asks why Plato thinks that ourthought and speech must, at a fundamental level, agree with reality.

Even if we find genuinely appealing Natorp’s interpretation of the Socraticmethod of searching for definitions, as being a search for the nature of rationalthought and speech from within such thought and speech alone, we may wonderwhether the same line of interpretation is appropriate for Plato’s theory of forms.The problem is that the forms do not appear to be elements in thought, or the natureof thought, but rather appear simply to be elements in reality, or the nature of reality.On this view, forms may be objects of rational thought, just as sense-perceptiblethings are objects of sense-perception, but their essence makes no reference tothought or the nature of thought. This problem is accentuated if we agree withAristotle that Plato conceived of the forms as, essentially, objects that are separateand distinct from the ones in nature with which we are directly familiar throughsense-perception. Natorp’s response is to argue that the theory of forms must not bedissociated from the Socratic method but must, on the contrary, be understood asgrowing naturally and inevitably out of this method. This leads him to characterizethe relation between the Socratic method and the theory of forms as follows:

There can be no doubt whatsoever that Plato’s ideas, from beginning to end [. . .] signify methodsand not things: units of thought, pure positings in thought and not external, albeit super-sensible,“objects”. It is nevertheless true that, in his “ideas”, Plato effected a separation, a freeing ofconcepts from the sense-perceptible, of which Socrates, though not wholly a stranger to the matter,was not conscious with such methodical rigour. (pp. 74–75)

For concepts are now [i.e., in the theory of forms as opposed to the Socratic method] to be graspedin thought as they are in their purity and in separation from everything that is sense-perceptible; andin the same manner the positing in thought is to be grasped purely and according to the content thatis posited, without any extraneous admixture. (p. 71)

So there is, Natorp argues, certainly a development from the Socratic method ofsearching for definitions to the theory of forms, and indeed this development

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consists, in a particular sense, in separating the forms from what is sense-perceptible. But this, he argues, is not a separation of two sets of objects, but aseparation of two elements in knowledge:

But the purely logical sense of all this [i.e., Plato’s claim that forms are separate from sense-perceptibles] can only be found, once we bear in mind the non-metaphorical explanations thatfollow, in the pure separation of the content that is present in thought and that is originally positedthrough thought, as, for instance, unity, identity, and hence being. (p. 72; see also his comments onTheaetetus 184–87, e.g., pp. 112–13)

So, on the one hand, there is the element in knowledge that is associated with thevery nature and conditions of rational thought and speech—such as the elements,unity, identity, being, etc. This is, precisely, the forms, or at any rate the mostabstract and general ones. On the other hand, there is the element in knowledgethat is associated with the sensory content of knowledge.

Thus, it emerges that the forms are, precisely, elements in the nature of rationalthought and speech, and in particular the kind of thought and speech that aims atknowledge. This, Natorp argues, is especially evident if we consider dialoguessuch as the Theaetetus and the Sophist and the role of certain most abstract andgeneral forms or concepts here, such as the koina, or common concepts, of theTheaetetus: “being and not-being, identity and difference, qualitative similarityand dissimilarity, unity and number, even and odd” (p. 112; see Theaetetus185–86). For here, Natorp emphasizes, Plato expressly argues that the function ofthe most general forms or concepts is to explain how judgment (doxa) and speech(logos) is possible. Commenting on Theaetetus 184–87, he says:

Since all possibility of judging, all determinate sense of statements, is grounded in them [these mostuniversal concepts], it is only through them, and not through sensory experience, that there can besuch a thing as being (in the widest sense of the content of statements), hence truth and henceknowledge. (p. 113)

Commenting on the role of “the most general kinds” (ta megista gene) and their“interweaving” (sumploke) in the Sophist, he concludes: “The justification of thevalidity of the ideas is to be based in the establishing of the conditions for thepossibility of judging in general” (p. 280). This, he thinks, is exactly what Platomeans when he says: “[F]or it is by virtue of the interweaving of the kinds (ta eide)with each other that speech (logos) comes to be for us” (Sophist 259e5–6). Notunnaturally, Natorp goes as far as characterizing these most general kinds, forms,or concepts as “categories” (see e.g., pp. 114–15, 292, 298, 304). He means“categories” in the Kantian sense of concepts whose function it is to account forthe possibility of judgment and predication.

For Natorp, then, the culmination of the theory of forms is to be found in Plato’sidentification, in such dialogues as the Theaetetus and the Sophist, of certain most

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general concepts whose function it is to explain how judgment (doxazein) andspeech (legein) is possible and in general to provide an account of the nature ofrational thought and speech. The search for this account, he argues, goes back tothe Socratic method of searching for definitions and in particular to the principlesthat guide this search. Furthermore, the search for an account of the nature ofrational thought and speech distinguishes the theory of forms from start to finish.Thus, as far back as the Phaedo, in a passage which is commonly thought to markPlato’s introduction of the theory of forms and his going beyond the Socraticsearch for definitions (Phaedo 99e–100a), the theory of forms is introducedagainst the background of the claim that we must search for knowledge notdirectly in things but through logoi. This claim occupies a central position inNatorp’s transcendental idealist interpretation, that is, the thesis that the nature ofreality is to be derived from the nature of rational thought and speech—from thenature of logos in this sense.

In Kant, the method of transcendental argument, that is, of first searching for theconditions of thought and knowledge and then concluding that reality is subject tocorresponding conditions, expressly forgoes any aspiration to ontology, that is, toproviding an account of the ultimate nature of reality (see e.g., Critique of PureReason, A247/B303). But the aim of Plato’s transcendental method, Natorpargues, is indeed to provide an account of the ultimate nature of reality and toderive this nature from the nature of thought and knowledge. There is no tensionin Plato between the transcendental method and ontology, and the two are, on thecontrary, perfectly matched.

This perfect match between the nature of thought and knowledge and the natureof reality is achieved, Natorp argues, because Plato thinks that the very concept of“being” and “what is real” cannot signify anything independent of, and not basedin, thought and knowledge:

“Being” means here, as it always does in Plato’s more rigorous philosophical language, the positingin thought, the unity of determination, and, therefore, of predication. (p. 103)

Being in general signifies only the function of judgment and has no ascertainable meaning apartfrom this. (p. 150)

It follows that objects, existing things, are nothing but logical subjects of thoughtand judgment; indeed to be an object is to be a logical subject of thought andjudgment. This view, Natorp emphasizes, means that the content of thought andjudgment is primary whereas the object referred to in thought and judgment isderivative, simply an element, or what is posited as real in an element, of thoughtand judgment. This stands in stark contrast to the view, abstractionism, whichholds that objects are primary and that concepts must, somehow, be derived fromour causal, sensory acquaintance with objects, a view that Natorp ascribes to

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Aristotle. More generally, Natorp here shows himself to be acutely sensitive to themanifold meaning of the verb “to be” in Greek, and especially to the distinctionbetween the existential (“to exist”) and the predicative (“to be such and such”)uses of this verb. In fact, he goes a step further and considers which of these twouses was seen as philosophically the more fundamental. For he argues that Platothinks we must understand existential being in terms of predicative being whereasAristotle thinks the converse is true.

In this sense, that is, in the sense that objects are simply what is posited as realin thought, objects are, as Natorp does not tire of saying, “constituted” by thoughtand knowledge:

The form of knowledge as such is lawfulness; but it is this form that constitutes the content, the purecontent of knowledge; for it is in general the law that creates the object in and for knowledge. Thisis the ultimate meaning of the “idea.” (p. 29)

There is no longer such a thing as a true object that is not constituted within the concept of knowledge,in accordance with the law proper to knowledge. Knowledge, pure knowledge, is the self-generatedconcept, in which alone the object becomes certain to us. The law peculiar to consciousness is whatgenerates the object in the first place, namely as object of consciousness. (p. 31)

This means, in particular, that knowledge must not be understood in terms of acausal relation between thought and reality, whether such a relation is conceivedas sensory and directed at sense-perceptible things or it is conceived as intellectualand directed at non-sense-perceptible forms. Rather, it must be understood as aprocess, indeed a process that is in principle infinite, of thought’s examination ofitself—as reflexive knowledge and self-knowledge in this sense.

III.4. How Credible Is Natorp’s Transcendental Idealist Interpretation of theTheory of Forms?

How, if at all, can Natorp’s transcendental idealist thesis contribute to ourunderstanding of Plato’s theory of forms and of his view of the relation betweenthought and reality? The question that Natorp’s thesis prompts us to ask is, first,whether transcendental arguments occupy any position in Plato, and if so, howcentral a position they occupy. And second, whether Natorp is right to deny thatPlato’s account of knowledge is a causal account, that is, an account in terms ofthe causal affection of things on either our senses or (directly or indirectly) ourintellect. This second question is important in view of Trendelenburg’s combina-tion of transcendental arguments with a causal realist account of unqualifiedlyobjective knowledge and in view of my suggestion that a similar combination canbe found in Aristotle’s account of the relation between thought and knowledge. Ishall, however, set aside this question for present purposes and focus exclusivelyon the first.

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It is notoriously difficult, with regard to any particular argument in Plato, tosay how central a place it occupies in the corpus—this is difficult not leastbecause of the dialogical context and indeed framework. But I think we canagree that, if any one argument is central, this will include the argument for thereality of change and changelessness (kinesis and stasis) in the Sophist (248e7–249d5).12 And this argument, properly understood, can I think be shown to be athoroughly transcendental argument: an argument that seeks to derive certainmost fundamental features of reality from certain most fundamental features ofthought and knowledge.

Certainly this argument plays a pivotal role in the Sophist. For it serves toestablish the two central of the five so-called “most important/largest kinds”(megista gene), the other three being: being, sameness, and difference. Changeand changelessness are central because it is due to the thesis that they are real,without however being identical with being real, that the kind being must beintroduced, and it is due to the thesis that these three kinds are all real, but identicalwith themselves and different from each other, that the kinds sameness anddifference must be introduced. And in the dialogue these five kinds provide thebasis for a positive account of what it means to say that something is and what itmeans to say that something is not. Moreover, the account of being and not-being,formulated in terms of the five most general kinds, is perhaps the most succinctand comprehensive account of a complete ontology, in the sense of an answer tothe question “What is there?” that Plato ever gives us. Here is Plato’s argument forthe reality of change and changelessness:

[1] 248e7 Eleatic Stranger (ES): In Heaven’s name! Are we really to be so easily persuaded thatchange, life, soul and intelligence are not present in complete being (to pantelos on), and that thisis neither alive nor intelligent but rather, august and holy, is without reason and changeless?

Theaetetus (THT): That would certainly be a terrible statement to admit.

ES: But are we to say that although it has reason, it does not have life?

THT: Of course not.

ES: But if we say that both these things are present in it, must we not say that it has them in a soul?

THT: How else would it have them?

ES: But if we say that it has reason, life, and soul, can we still say that, though it is ensouled, it isutterly changeless?

THT: That seems entirely unreasonable to me.

ES: But then we must acknowledge as beings that which changes and likewise change.

12 For an analysis of this argument, see my “The Argument for the Reality of Change and Change-lessness in Plato’s Sophist (248e7–249d5),” New Essays on Plato, ed. Fritz-Gregor Herrmann(Swansea: Classical Press of Wales) 149–74.

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THT: Certainly.

ES: So it turns out, Theaetetus, that if all things are changeless, there is no such thing as reasonanywhere about anything.

THT: Exactly.

[2] 249b8 ES: But if, on the other hand, we admit that all things are moving and changing, then,according to this claim, too, we will be excluding the very same thing from the things that are.

THT: How so?

ES: Do you think that without changelessness there could ever be such a thing as staying the samein the same condition and in the same respects?

THT: Not at all.

ES: But without this, can you make out that there is such a thing as reason anywhere, or could everbe?

THT: Not in the least.

ES: And we need to oppose with all the force of reasoning the person who in any way maintainsanything about anything while doing away with knowledge, intelligence and reason.

THT: Most definitely.

[3] 249c10 ES: From all this it follows, as it appears, that the philosopher, that is, he who supremelyvalues all these things, must refuse to accept, from the defenders of either the One or the manyforms, that the totality of things is changeless, and likewise he must turn a deaf ear to those whomake being change throughout, rather, like a child begging for both, he must declare that being andthe totality of things is as many things as are changeless and as many things as are changing, bothtogether.

THT: Perfectly true.

How, then, does Plato argue for the reality of change and changelessness in thisargument, which is addressed to the so-called “friend of the forms” (248a)? Theargument for the reality of changeless things (akineta, stasima) and changeless-ness (stasis) is, expressly, that unless we suppose that they are real, we mustsuppose that reason, intelligence, and knowledge are impossible—and this issuesin a performative contradiction. And it seems clear from these lines that thereason, intelligence, and knowledge in question is our reason, intelligence, andknowledge, insofar as we are prepared to “maintain anything about anything,” thatis, to make assertions. That our reason is in question is clear also from theimmediate context, since immediately prior to the argument reasoning was iden-tified as one of the functions of our soul, the other being sense-perception (248a).It is also notable that the conclusion of the argument is formulated in terms ofwhat a philosopher—a searcher for knowledge—must commit himself to as beingreal. The nub of the argument, we may suppose, is that things, insofar as they areknowable, must be changeless (for the “insofar,” kath’ hoson, cf. 248e3). Presum-ably, this is because properly to know a thing one must know what it is, its essence,

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and the essence of any thing is changeless (or at least, properly to know a thing onemust know the essence of either that thing or of another thing on which itontologically depends). Since the argument is addressed to the friends of theforms, the friends of changeless things par excellence, we may suppose that thisis an argument for the reality of forms and the changelessness peculiar to them. Ifso, the argument derives a fundamental element of reality—changeless things andchangelessness—from the possibility of knowledge, the knowledge that we areseeking. The crux of the argument is that knowledge must be of something, it musthave an object, and it seeks to establish a fundamental feature of the object ofknowledge from the very possibility of knowledge.

Let us, next, turn to the argument for the reality of changing things (kinoumena)and change (kinesis). Remarkable is the express claim that “if all things arechangeless, there is no such thing as reason anywhere about anything” (readtogether with what immediately follows: “But if, on the other hand, we admit thatall things are moving and changing, then, according to this claim, too, we will beexcluding the very same thing from the things that are”). Why should the posses-sion of knowledge require the reality of changing things and change? Apparently(based on a natural reading of what leads to this conclusion, from 247e7), becausereason, intelligence, and knowledge belong to a soul, and a soul is a changingthing. The soul in question is, it appears, our soul (see above). But why should oursoul be something changing insofar as it has intelligence, reason, and the capacityof rational knowledge? Presumably, because searching for knowledge is a par-ticular kind of change, since it is an activity, and/or the acquisition of knowledgeis a kind of change, since it is a process. If so, the argument derives a furtherfundamental element of reality, the change peculiar to the rationally cognizingsoul, from the possibility of knowledge, the knowledge that we are searching for.The crux of the argument is that knowledge must be by something, it must havea subject, and the argument seeks to establish a fundamental feature of the subjectof knowledge from the very possibility of knowledge.

Evidently, thus understood, the two arguments—for the reality of stasis and forthe reality of kinesis—are perfectly complementary, for knowledge requires botha subject and an object. Indeed, Plato points out this mutual dependence of“knowing” (to gignoskein) and its subject, the soul, on the one hand, and the thingknown (to gignoskomenon) immediately prior to our argument, on the other (seeesp. 248d–e).

I recognize, of course, that this interpretation of the argument, as being athoroughly transcendental argument, requires a good deal of further defense. ButI hope we can agree that the interpretation does not seem forced, on the contraryit seems natural. A proper defense of this interpretation would require addressingthe following points. First, the interpretation requires taking the change in ques-tion here to include only the change peculiar to the rationally cognizing soul and

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not also physical change. For not even Plato could hope to establish the reality ofphysical change purely by reference to the possibility of knowledge and withoutreference to the datum of sense-perception. Second, it requires taking the aim ofthis argument to be the establishment of what belongs to to pantelos on (“completebeing”, Sophist 248e8-249a1) in the sense of “perfect being” and not in the senseof “all-inclusive being”; that is, this phrase must be taken to refer not to the wholeof being, but exclusively to the “upper” tier of being: the tier that includeschangeless forms and, on this argument, rationally cognizing souls. For if topantelos on means rather “all-inclusive being,” then clearly the change in questionmust include physical change. Finally, in order to trace wider implications of theargument, in the way in which I am trying to do, we must suppose that theso-called “friends of the forms” include Plato himself. That is, Plato wouldinclude himself among those philosophers who believe in forms and conceive offorms of as perfect beings—primary beings. His point of contention here iswhether primary being (to pantelos on) includes only changeless forms and thechangelessness peculiar to them or it includes also the rational soul and the changepeculiar to it, which latter is what he here defends.

Trinity College Dublin

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