1. Skepticism

2. Skepticism
The Skeptics were one of the great branches of Greek philosophy (after Plato and Aristotle)
Inspired by the problem of justifying belief
Only certainty counts as knowledge
No justification can be strong enough to create certainty
Two types:
Academic – the position of the later Academy
Pyrrhonian – after Pyrrho of Elis

3. Skepticism Objections to absolute doubt
Academic position is self refuting:
I know that nothing is knowable (???)
Pyrrhonian position is a non-position
We should always withhold judgement on any question
Still, is that a true statement?

4. Skepticism Objections to absolute doubt Incoherent:
Knowledge and doubt are polar concepts. You need one to make sense of the other.
To say there is no knowledge means there is no concept of doubt, so there’s no concept of knowledge, so you can’t say anything sensible about it

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

6. Decartes’s Project 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.

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

8. The Method of Doubt 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
Therefore reject X
This is his Method of Doubt

9. The Method of Doubt Senses are the root of most ideas and knowledge
But:
... 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

10. The Method of Doubt Dreams may appear to give sense data But
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
= 5 and a square has 4 sides even in a dream

11. Ancient Interlude
St Augustine objected to skepticism for most of the same reasons
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.

12. The Method of Doubt
God, or a powerful Demon, may deceive us about even these most certain things, so that our certainty that = 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.

13. All that we know is:
Descartes thinks he can reconstruct a structure of knowledge about the world beginning with one fundamental undoubted truth
I think, I am
Cogito, ergo sum

14. All that we know is: Augustine had the same thought
…who would doubt that he lives, remembers, understands, wills, thinks, knows, and judges? For even if he doubts, he lives; if he doubts, he remembers why he doubts; if he doubts, he understands that he doubts; if he doubts, he wishes to be certain; if he doubts, he thinks; if he doubts, he knows that he does not know; if he doubts, he judges that he ought not to consent rashly. Whoever then doubts about anything else ought never to doubt about all of these…