Varieties of ScepticismVarieties of Scepticism

Academic ScepticismAcademic Scepticism

Arcesilaus, 6Arcesilaus, 6thth scolarch of the scolarch of the AcademyAcademy A return to the Socratic and early A return to the Socratic and early

Platonic Platonic aporeticaporetic approach approach Opposed to dogmatic systemsOpposed to dogmatic systems

Aristotle, Stoics, …Aristotle, Stoics, …

Academic ScepticismAcademic Scepticism

Critique of the sensesCritique of the senses Any sense impression Any sense impression might might be be

veridical; or veridical; or It might be caused by something other than It might be caused by something other than

it appeared to beit appeared to be Twins, eggs, …Twins, eggs, …

It might not represent anything at allIt might not represent anything at all Winged horses, dragons, …Winged horses, dragons, …

No sense impression can be trustedNo sense impression can be trusted

Academic ScepticismAcademic Scepticism

Devastating for Aristotle’s systemDevastating for Aristotle’s system If sense impressions can’t be trusted, thenIf sense impressions can’t be trusted, then General statements found by induction on General statements found by induction on

sense data can’t be trusted, and sense data can’t be trusted, and Deductions using those general Deductions using those general

statements can’t be trustedstatements can’t be trusted Since scientific knowledge is a system of Since scientific knowledge is a system of

deductions from inductively discovered deductions from inductively discovered principles, the whole thing will collapse if principles, the whole thing will collapse if sense data are removed sense data are removed

Academic ScepticismAcademic Scepticism

What can the sceptic say?What can the sceptic say? Can’t say ‘We know nothing’ since that’s a Can’t say ‘We know nothing’ since that’s a

statement of knowledgestatement of knowledge Recommends a Recommends a suspension of suspension of

judgementjudgement No knowledge claims can be madeNo knowledge claims can be made Their investigations (Their investigations (skepsesskepses) showed all the ) showed all the

ways that both sides of an argument can be ways that both sides of an argument can be putput

This was supposed to make suspension of This was supposed to make suspension of judgement easier to justify in particular casesjudgement easier to justify in particular cases

ProblemsProblems

Living skeptically is a problemLiving skeptically is a problem How can we do anything at all if we How can we do anything at all if we

don’t believe anything?don’t believe anything? This is a problem, since Greek This is a problem, since Greek

philosophy was supposed to be philosophy was supposed to be ultimately a guide to the good lifeultimately a guide to the good life

ProblemsProblems

Living skeptically is a problemLiving skeptically is a problem

Arcesilaus asserts that he who suspends judgment about everything will regulate his inclinations and aversion and his actions in general by the rule of “the reasonable,” and by proceeding in accordance with this criterion he will act rightly; for happiness is attained by means of wisdom, and wisdom consists in right actions, and the right action is that which, when performed, possesses a reasonable justification. He, therefore, who attends to “the reasonable” will act rightly and be happy

ProblemsProblems

It denies things we It denies things we dodo know know St Augustine pointed out that St Augustine pointed out that

I still know something about physics. For I am certain that (1) there is either one world or not. And (2) if there is

not just one, the number of them is either finite or infinite… In the same way, I know that (3) our world is disposed as it is either by the nature of bodies or by some plan. And I know that (4) (a) either it always did exist and always will, or (b) it started to exist and will never stop, or (c) it did not start in time but will have an end, or (d) it started and will not last forever…These truths are disjunctions, and no one can confuse a likeness of something false with them

ProblemsProblems

It denies things we It denies things we dodo know know St Augustine also pointed out that we St Augustine also pointed out that we

know mathematical and logical truthsknow mathematical and logical truths Don’t we Don’t we know know that 2 + 2 = 4?that 2 + 2 = 4? Don’t we Don’t we know know that ‘everything is either A that ‘everything is either A

or not A’?or not A’?

ProblemsProblems

It denies things we It denies things we dodo know know St Augustine finally noted that we St Augustine finally noted that we know know

we are in the worldwe are in the world

…who would doubt that he lives, remembers, understands, wills, thinks, knows, and judges? For even if he doubts, he lives

DescartesDescartes

Born 1596, la-Haye-Descartes. Born 1596, la-Haye-Descartes. Minor noble family. Minor noble family. Died 1650, Stockholm Died 1650, Stockholm

Text: Text: MeditationsMeditations

Decartes’s ProjectDecartes’s Project

What do we What do we really really knowknow

For several years now, I have been aware For several years now, I have been aware that I accepted many falsehoods as true that I accepted many falsehoods as true in my youth, that what I had built on the in my youth, that what I had built on the foundations of those falsehoods was foundations of those falsehoods was dubious, and therefore that, once in my dubious, and therefore that, once in my life, I would need to tear down everything life, I would need to tear down everything and begin anew from the foundations if I and begin anew from the foundations if I wanted to establish any firm and lasting wanted to establish any firm and lasting knowledge. knowledge.

Decartes’s ProjectDecartes’s Project

Destroy the most basic beliefs firstDestroy the most basic beliefs first Our beliefs are a vast structure of Our beliefs are a vast structure of

related ideasrelated ideas Remove the bases of the structure and Remove the bases of the structure and

the rest will collapsethe rest will collapse

The Method of DoubtThe Method of Doubt

Destroy the most basic beliefs firstDestroy the most basic beliefs first If there is some way, no matter how If there is some way, no matter how

absurd, that I may be wrong about X, absurd, that I may be wrong about X, then I can’t then I can’t knowknow X X

Therefore reject XTherefore reject X This is his This is his Method of DoubtMethod of Doubt

The Method of DoubtThe Method of Doubt

SensesSenses are the root of most ideas and are the root of most ideas and knowledgeknowledge

But:But:

... I have occasionally caught the senses deceiving me, ... I have occasionally caught the senses deceiving me, and it would be prudent for me never completely to trust and it would be prudent for me never completely to trust those who have cheated me even once. those who have cheated me even once.

And yet:And yet:

there may still be other things taken in by the senses there may still be other things taken in by the senses which I cannot possibly doubt — such as that I am here, which I cannot possibly doubt — such as that I am here, sitting before the fire, wearing a dressing gown, touching sitting before the fire, wearing a dressing gown, touching this paperthis paper

The Method of DoubtThe Method of Doubt

DreamsDreams may appear to give sense data may appear to give sense dataButBut

When I think very carefully about this I see so plainly When I think very carefully about this I see so plainly that there are no reliable signs by which I can that there are no reliable signs by which I can distinguish sleeping from waking that I am stupefied distinguish sleeping from waking that I am stupefied ……

And yetAnd yet

1.1. colours, shapes, extension, etc. must have some colours, shapes, extension, etc. must have some sensory sensory basis to exist as conceptsbasis to exist as concepts

2.2. 2 + 3 = 5 and a square has 4 sides even in a 2 + 3 = 5 and a square has 4 sides even in a dreamdream

The Method of DoubtThe Method of Doubt

God, or a powerful God, or a powerful DemonDemon, may deceive , may deceive us about even these most certain us about even these most certain things, so that our certainty that 2 + 3 things, so that our certainty that 2 + 3 = 5 is no guarantee that it is so.= 5 is no guarantee that it is so.

I will suppose then, not that there is a supremely I will suppose then, not that there is a supremely good God who is the source of all truth, but that good God who is the source of all truth, but that there is an evil demon, supremely powerful and there is an evil demon, supremely powerful and cunning, who works as hard as he can to deceive me. cunning, who works as hard as he can to deceive me. I will say that sky, air, earth, color, shape, sound, and I will say that sky, air, earth, color, shape, sound, and other external things are just dreamed illusions other external things are just dreamed illusions which the demon uses to ensnare my judgement. which the demon uses to ensnare my judgement.

ReconstructionReconstruction

Descartes thinks he can reconstruct Descartes thinks he can reconstruct a structure of knowledge about the a structure of knowledge about the world beginning with one world beginning with one fundamental undoubted truthfundamental undoubted truth I think, I amI think, I am Cogito, ergo sumCogito, ergo sum