1. Varieties of Scepticism

2. Academic Scepticism Arcesilaus, 6 th scolarch of the Academy Arcesilaus, 6 th scolarch of the Academy A return to the Socratic and early Platonic aporetic approach A return to the Socratic and early Platonic aporetic approach Opposed to dogmatic systems Opposed to dogmatic systems Aristotle, Stoics, … Aristotle, Stoics, …

3. Academic Scepticism Critique of the senses Critique of the senses Any sense impression might be veridical; or Any sense impression might be veridical; or It might be caused by something other than it appeared to be It might be caused by something other than it appeared to be Twins, eggs, … Twins, eggs, … It might not represent anything at all It might not represent anything at all Winged horses, dragons, … Winged horses, dragons, … No sense impression can be trusted No sense impression can be trusted

4. Academic Scepticism Devastating for Aristotle’s system Devastating for Aristotle’s system If sense impressions can’t be trusted, then If sense impressions can’t be trusted, then General statements found by induction on sense data can’t be trusted, and General statements found by induction on sense data can’t be trusted, and Deductions using those general statements can’t be trusted Deductions using those general statements can’t be trusted Since scientific knowledge is a system of deductions from inductively discovered principles, the whole thing will collapse if sense data are removed Since scientific knowledge is a system of deductions from inductively discovered principles, the whole thing will collapse if sense data are removed

5. Academic Scepticism What can the sceptic say? What can the sceptic say? Can’t say ‘We know nothing’ since that’s a statement of knowledge Can’t say ‘We know nothing’ since that’s a statement of knowledge Recommends a suspension of judgement Recommends a suspension of judgement No knowledge claims can be made No knowledge claims can be made Their investigations (skepses) showed all the ways that both sides of an argument can be put Their investigations (skepses) showed all the ways that both sides of an argument can be put This was supposed to make suspension of judgement easier to justify in particular cases This was supposed to make suspension of judgement easier to justify in particular cases

6. Problems Living skeptically is a problem Living skeptically is a problem How can we do anything at all if we don’t believe anything? How can we do anything at all if we don’t believe anything? This is a problem, since Greek philosophy was supposed to be ultimately a guide to the good life This is a problem, since Greek philosophy was supposed to be ultimately a guide to the good life

7. Problems Living skeptically is a problem Living 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

8. Problems It denies things we do know It denies things we do 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

9. Problems It denies things we do know It denies things we do know St Augustine also pointed out that we know mathematical and logical truths St Augustine also pointed out that we know mathematical and logical truths Don’t we know that 2 + 2 = 4? Don’t we know that 2 + 2 = 4? Don’t we know that ‘everything is either A or not A’? Don’t we know that ‘everything is either A or not A’?

10. Problems It denies things we do know It denies things we do know St Augustine finally noted that we know we are in the world St Augustine finally noted that we know we are in the world …who would doubt that he lives, remembers, understands, wills, thinks, knows, and judges? For even if he doubts, he lives

11. Descartes Born 1596, la-Haye-Descartes. Born 1596, la-Haye-Descartes. Minor noble family. Minor noble family. Died 1650, Stockholm Died 1650, Stockholm Text: Meditations Text: Meditations

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

13. Decartes’s Project Destroy the most basic beliefs first Destroy the most basic beliefs first Our beliefs are a vast structure of related ideas Our beliefs are a vast structure of related ideas Remove the bases of the structure and the rest will collapse Remove the bases of the structure and the rest will collapse

14. The Method of Doubt Destroy the most basic beliefs first Destroy the most basic beliefs first If there is some way, no matter how absurd, that I may be wrong about X, then I can’t know X If there is some way, no matter how absurd, that I may be wrong about X, then I can’t know X Therefore reject X Therefore reject X This is his Method of Doubt This is his Method of Doubt

15. The Method of Doubt Senses are the root of most ideas and knowledge Senses are the root of most ideas and knowledgeBut:... I have occasionally caught the senses deceiving me, and it would be prudent for me never completely to trust those who have cheated me even once. And yet: there may still be other things taken in by the senses which I cannot possibly doubt — such as that I am here, sitting before the fire, wearing a dressing gown, touching this paper

16. The Method of Doubt Dreams may appear to give sense data Dreams may appear to give sense dataBut When I think very carefully about this I see so plainly that there are no reliable signs by which I can distinguish sleeping from waking that I am stupefied … And yet 1.colours, shapes, extension, etc. must have some sensory basis to exist as concepts 2.2 + 3 = 5 and a square has 4 sides even in a dream

17. The Method of Doubt God, or a powerful Demon, may deceive us about even these most certain things, so that our certainty that 2 + 3 = 5 is no guarantee that it is so. God, or a powerful Demon, may deceive us about even these most certain things, so that our certainty that 2 + 3 = 5 is no guarantee that it is so. I will suppose then, not that there is a supremely good God who is the source of all truth, but that there is an evil demon, supremely powerful and cunning, who works as hard as he can to deceive me. I will say that sky, air, earth, color, shape, sound, and other external things are just dreamed illusions which the demon uses to ensnare my judgement.

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