Philosophy

Skepticism is as about as old as is philosophy. In the second century, Sextus Empiricus laid down the 'tropes' (modes of skeptical argument) which included much of which occupies contemporary philosophers: disagreement, regress, relativity, hypothesis and circularity . The contemporary skeleton of the skeptical argument was laid down by Descartes: sense-data is unreliable, we might be dreaming, we might be being manipulated by an evil demon. Hume said 'I am ready to reject all belief and reasoning, and can look upon no opinion even as more probable or likely than another ... ' and Kant claimed 'it remains a scandal to philosophy ... that the existence of things outside of us ... must be accepted merely on faith ... we are unable to counter their [the skeptics] doubts with any satisfactory proof.

His argument tacks between the modal logic of knowledge propositions and pragmatic 'rules' that tell us what possibilities we can safely ignore. For example, the 'rule of actuality' says we can't ignore what actually happens; the 'rule of belief says we can't ignore what we believe; the 'rule of resemblance' says we can't ignore possibilities that resemble possibilities that we are considering.

Putnam considers the meaning or 'intentionality' of what the statement 'I am a brain in a vat' would be if we were actually brains in vats. If we were, he asserts, we would have no image of a brain or a vat. Similarly, the Twin Earth argument suggests that if on an identical planet, but one where water was known by the chemical formula of XYZ, the word water might refer to some common properties (for example 'wetness'), but the reference would be different in an important way.

Nagel challenges Putnam's 'reference' argument, saying that we can use terms that do not refer, provided there are conditions under which the ...