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On the Theory of Visual Perception Kepler and Descartes." Reflections on the Role of Mechanism in the Birth of Modern Science GI~RARD SIMON

Universitd de Lille, France

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WE SHALL consider by way of example the theory of vision, and try to isolate some elements from it to help us to understand the necessary role played by mechanism in the origins of modern science.

In Astronomiae Pars Optica, Chapter V (1604), Kepler had obtained definitive results for his theory of vision. Starting with the geometrical study of the camera obscura he established rigorously what Porta had guessed at as early as 1589 in his Magia Naturalis. Kepler uses this device to prove that, contrary to the ancient theories that rays are emitted by the eyes, light rays indeed come from the object and then enter the eye? In addition, he was the first to explain the function of the crystalline lens, which had previously been thought to be the place where the image was formed, and the specific organ for visual sensation. Porta, who in this point too followed the tradition of Alhazen and Vitellion, saw the lens as the analogue of the screen which receives the image formed in the camera obscura. Kepler, on the other hand, established that it functions in the manner of a lens, or more exactly of a

1 j. B. Porta, Magia Naturalis. Edition of 1589. Salviani, Napoli, Book XVIII, Chap. VI. (This text is not to be found in the earlier editions.)

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spherical diopter placed behind the opening of the camera. Using his previous study of refraction in Chapter IV, in Chapter V he discusses the importance of small angles of incidence, 2 the role of the diaphragm 3 and the necessity of stigmatism 4 in obtaining sharp images. The optical system formed by the pupil, the aqueous humor, the crystalline lens and the vitreous humor uses a diaphragm and two refractions to produce an image at the back of the eye. Thus, although he had not succeeded in discovering the law of refraction, Kepler was the first to establish the existence of the retinal image and to describe how it was formed.

On all these points, Descartes in his Dioptrique (1637) adds only details and con- firmation. Knowing the law of refraction he confirms the analogy of the crystalline lens and a converging lens; also by cutting through the posterior part of the sclerotic and replacing it with an eggshell he demonstrates the image formed on the back of the eye. 5 Finally, he gives a more accurate anatomical description of the eye: instead of placing the entry of the optic nerve on the axis of the optical system of the eye, as did Alhazen, Vitellion and Kepler (who was following two recent medical descriptions), he moves it to its correct position. 6

This anatomical modification brings us to the heart of the matter, because it is not fortuitous: it was essential to the pre-Cartesian theories of vision that the optic nerve should open up right in the middle of the back of the eye, and when it is displaced the centre of the analysis of the problem is displaced with it. We should indeed note that nearly half of Descartes' treatise, from Discourse III to Discourse VI I, is devoted to what we should now call the anatomy and the psycho-physiology of vision. The discovery of the retinal image meant that it was no longer possible to avoid dealing with a difficult problem: how does this image reach the brain, and, which is even more important, how does it become a sensation ?

The question could remain dormant in the early work, that of Alhazen (Ibn-al- Haitham 965 ?-1039) and Vitellion (Vitello 1220-1230 ? to 1275 ?). Those early dates must not mislead us: the works were printed from manuscripts in Basle in 1572 at the request of Ramus, and at the end of the sixteenth century provided the most complete and perfectly up-to-date treatises on geometrical optics. Medieval realism in fact covered up the problem of transmission through the nerves: as Kepler sees very clearly 7 the epistemological difficulty Vitellion ran up against in attempting to under- stand the role of the crystalline lens was that behind it images would be upside down;

j. Kepler, Ad Vitellionem Paralipomena, quibus Astronomiae Pars Optica traditur, Chap. V, 3; Prop. 15. (Vol. 2 of the edition in Gesammelte Werke, Beck, Munich, 1939, to which all our references refer.)

8 j. Kepler, Ad Vitellionen, etc., Prop. 23. J. Kepler, Ad Vitellionen, etc., Prop. 24.

+ Oeuvres de Descartes, Vol. VI. Vrin, Paris, 1969. (Revised Edition by Adam and Tannery.) Dioptrique, Discours V. Des images qui se forment sur le fond de l'ofil.

Dioptrique, Discours III, p. 106; De l'oeil. 7 j. Kepler, op. cit., V, 2, p. 152. See also V, 4, p. 185.

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he thought they had to be formed upright and transmitted upright to the seat of the visual faculty (Fig. 14.4). Situated in the centre of the eye, which it divides into an optical and a sensory part, the crystalline lens has the role of mediator between two domains, the external and the internal, both conceived in the same way. Light carries the visible form to the crystalline lens, where the image is formed using a series of transparent "tunics"; the visual spirit (spiritus visorii) which travels through the vitreous humor, the crystalline lens and the optic nerve, in its turn carries it through media similar to the preceding ones to the optic chiasma, the seat of the visual faculty. The sensory process is a repeat of the optical process: to the extent that the optic nerve is represented as being wide, hollow, straight and along the axis of the eye, so that the image can go straight into it--as into a sort of funnel, as Vitellion puts it. 8 The essential point is that the object, through its image, should be summoned before the seat of the faculty concerned. 9 For the problem of perception is less physiological than logical and topological; it consists of analyzing and localizing the faculties which enable one to see. At the level of the crystalline lens, what is properly called the sense (sensus) recognizes only light and colour, which alone are by themselves "visible"; at the chiasma the visual faculty (facultas visoria) uses acquired knowledge (scientia) to recognize objects; in the brain the sensus communis uses reasoning (syllogismus) to make comparisons and judgements, for example, of distance or beauty. The optician need not account for our becoming conscious of the images, because the sensible is by definition already present to the sensory organ; but he must localize the faculties which led to this knowledge, and his problem is less that of discovering the transmission mechanism than that of establishing a geography of faculties. The repeat of the optical process in a sensory form between the crystalline lens and the chiasma does no more than translate into anatomical terms the analogy already supposed to exist between the sense and the sensible, allowing us to dispense with a consideration of the nature of the mental phenomenon.

With the discovery of the retinal image the situation changes. From now on we are acutely concerned with the question of its transmission to the brain, as Kepler clearly recognized:

How this portrait or this picture links up with the visual spirits dwelling in the retina and the nerve; whether it is by these spirits that it is led through the hollow passages of the brain to appear before the tribunal of the soul or of the visual faculty, or whether on the con- trary it is the visual faculty or deputy appointed by the soul which descends from the judgment hall of the brain into the optic nerve itself and into the retina, or even to the lowest benches of the tribunal, going out to meet this image; certainly I leave that to the physicians to decide (Fig. 14.5). l°

s Vitellionis Thuringopoloni Opticae libri decem. Book II I , Prop. 4, p. 185. (Vitellion's work was published by Risner, together with that of Alhazen, under the title Opticae Thesaurus Alhazeni Arabis libri septem, Basileae, 1572.)

9 ViteIlionis, etc., particularly Props. 6, 17 and 20. lo j . Kepler, op. cit., V, 2, pp. 151-2.

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FIG. 14.4. The structure of the eye. (From F. Risner's edition of Alhazen and Vitello: Opticae Thesaurus, Basle, 1572.)

FIG. 14.5. Sketches of the eye. From: Kepler, Gesammehe Werke, Vol. 2, p. 159, 1939.

On the Theory of Visual Perception

We would like to show that Kepler, thinking according to the traditional categories, is at once capable of formulating the problem, and incapable of resolving it.

The surprising thing for the modern reader is that he does not start either from the specific character of transmission by the nerves, or the mental character of the con- scious perception: neither the field of physiology nor that of psychology exists for him. He merely pushes to the limits the modes of thought of his time. He shows that where the nerve is concerned one is no longer dealing rpith an optical process: whether the internal channel of the nerve exists or not--and he inclines to believe it does 11- the "visual spirits" which transmit the image to the visual faculty or to the soul travel by a way too twisted, too narrow, and thus too dark for them to obey the laws of optics. It should be noted that in all these texts, as in many others, 1~ the pertinent opposition is that of bright and dark--that is of categories of concrete sensation--although the spiritus visorii can be affected by light and colour, as is proved by the persistence of the retinal image, 1~ they are able to travel through an opaque medium, and consequently are not of the nature of light. Kepler in no way abandons an Episteme (to use Michel Foucault's term) based on analogy and similarity; to be affected by light the visual spirits must like light be of the most subtle, and the inside of the nerve must therefore not be encumbered with any matter. 14 On the contrary Kepler's problem arises from this Episteme: how can something luminous and coloured like the image travel in the dark and opaque nerve ?15

The homology must thus be sought at a higher level: the homology between the soul and light. Light which is corporeal but immaterial (because it is weightless) is capable of spreading out in a sphere from its source without loss of any kind; it produces heat; in the same way the soul, which is related to it, 16 and also a source of animal heat, can also spread itself through space ;~v for instance, the soul of the Earth which spreads out spherically from its centre and, without the help of sensory organs, can recognize the directions of the luminous rays which move it. 18 Like acts upon like: the immaterial in space acts on the immaterial spread through space. The difference between the mental representation and the corporeal process causes no difficulty under these conditions; Kepler has no trouble in maintaining an objective geography of faculties analogous to that of Vitellion. The seat of the visual faculty may be the

11 j . Kepler, op. cit., V, 2, p. 157. 12 See the whole of J. Kepler, op. cit. V, 2. For example, p. 152: "What can one say about this

hidden crossing ['the chiasma', Editor's note] by considering the laws of optics ? As it is to be found in the opaque parts and thus in the darkness, it is controlled by the spirits, which are completely different in nature from the humors and the other transparent bodies, and it is thus not subject to the laws of optics."

13 See footnote 12. 14 j . Kepler, op. cit., V, 2, p. 157. 15 j . Kepler, op. cit., V, 2, Chap. I, Props. 1-9. le j . Kepler, op. cit., V, 2, Prop. 32, p. 35. 17 j . Kepler, Harmonice Mundi, IV, Chap. I, Gesammelte Werke, Vol. VI, pp. 223-5. 18 j . Kepler, Harmonice Mundi, IV, Chap. VII, p. 276.

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chiasma or, as for the soul, the brain; but the retina replaces the crystalline lens as the organ of sense. The difficulty arises not in understanding the action of light on the soul, but in understanding its journey through the body, which is its opposite in being opaque, gross and heavy.

How does it happen that Kepler formulates a problem which escaped his pre- decessors, since--if we may say so--he thinks in the same manner ? It comes from the rigour of his mathematical demands, which obliged him to make a mechanical translation of the representations used by his predecessors and thus showed up their inconsequence. From the moment it was discovered that the image is formed at the back of the eye, the repeat in sensory terms of the optical process worked out by Vitellion is incorrect and there is no other possible process. Kepler's genius thus gives us the opportunity of seeing a rare historical phenomenon: a mode of thought--the analogical one, so widespread in the sixteenth century--explores its own limits and although it cannot go beyond them clearly determines where they lie. That is the meaning of the challenge Kepler throws out to the "physicians", that is to say to the naturalists and the doctors of medicine.

As often happens, Descartes could only progress by completely altering the formu- lation of the problem, starting with its philosophical assumptions. His treatment of sensory qualities leads him to make a sharp distinction between mental representations and their material causes :19 the former cease to be the pictures or the analogues of the latter. The sensation ofa colour like red is caused by a modification in the transmitting region--a rotation of the little balls which compose it--and is in no way like the sensation itself; 2° more generally, the phenomenon of consciousness, which has nothing to do with space, has nothing in common with the corporeal process except for its coincidence in time. One is dealing with two different substances, the soul and the body, and nothing of what happens in one must be confused with what happens in the other. 21 Thus Dualism makes a radical break with the Episteme of similarities. This is one of the most direct and least stressed of the consequences of the Cartesian "Cogito". For our purposes it has two consequences. Firstly, light and the body have the same fundamental nature, both being matter subject to mechanical laws: the pressure of the medium which transmits the movement received from the light source can thus in turn move the filaments of the optic nerve; and the rays which form the image on the retina move these filaments, so causing an equivalent of the retinal image to be formed in the brain (Fig. 14.6). 22 The difficulty is obviously merely shifted because we now need to know how this "painting" in the brain produces a psychic

19 Descartes, Dioptrique, Discours V (Adam-Tannery), Vol. VI, p. 114: "Thus you can see clearly that in order to be affected, the soul need not contemplate any images which be similar to the things by which it is affected."

20 Descartes, Dioptrique, Discours, I, p. 92. 21 Descartes, Principles de la philosophie, I, article 61. Of modal distinction, Adam-Tannery,

Vol. VII, 1, p. 29 and IX, 2, p. 52. 22 Descartes, Dioptrique, V, Adam-Tannery, Vol. VI, pp. 128-9.

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FIG. 14.6. On the formation of visual images (1637). From: Oeuvres de Descartes (Adam-Tannery), p. 128.

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sensation, but--and this is the second consequence--this difficulty does not lie within the realm of physics, it is metaphysical: it raises the problem of the relationship be- tween soul and body. Descartes therefore contents himself with saying that the changes in the brain give occasion 23 to the soul to perceive the object, and it is only the contents of his sixth MJditation Metaphysique that justify this assertion.

What have we gained from the shifting of the problem ? Firstly, it is now purer. We can no longer speak of the physical in mental terms, nor of the spirit in corporeal ones. An end has been made, as Descartes himself says, of all the "species intentionales" of the Scholastics fluttering through the air; 24 and, one can add, of "spirits" of sight and of hearing, and more generally, of the occult qualities which according to a physical-psychical topology are scattered among organs and things. The demand of mechanism is pushed to the limit and implies that the human body itself shall be treated as a machine. Obviously, by reducing matter to extent and movement, and the living to the mechanical, Descartes impoverishes his object by reducing it, and this explains the short-lived nature of his results. But even if he underestimates the richness of the physical domain and the specific nature of physiology, he brings to an end the lumping together of the living and the non-living, and animate and the inanimate, which formed the basis of medical theories as well as the conceptions of alchemy and the astrological representations of the sixteenth century.

Let us go further. It is not only on the level of concepts that Descartes' Dualism, the corollary of his mechanism, imposed new demands. In fact, by putting it into words, he contributed to the establishment of a new mode of thought. After him the panpsychical and occultist naturalism of the sixteenth century falls into disuse and becomes esoteric. It is abandoned often even without needing to be refuted. For the endless play of analogies, of similarities and of value judgements which gave it its basis of categories has ceased to be plausible. Let us remember that even in 1609 a man of Kepler's stature asks himself in his great work the Astronomia Nova whether the planets have a soul to direct them, and only abandons this hypothesis because such souls would be incapable of calculating the complex elements of their trajectory. A few years later such hesitation is unthinkable. Descartes thus seems to us to have formu- lated a new type of demand for rationality, whose historical function has been to des- troy the animistic obstacles to the developments of modern science. This would explain the duration and universality of his influence on the science of his century, which is out of all proportion to the effective importance of his positive results. The problem of visual perception seems to us to be one of the nodal points which allow us to grasp the necessity and the scope of the intellectual and cultural change of which he was the theorist.

~8 Descartes, Dioptrique, V, Adam-Tannery, Vol, VI, p. 114. z4 Descartes, Dioptrique, Discours I, p. 85.

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