Debate: Can you know anything?

Radical skepticism

We do not know anything about the external world, because we cannot rule out scenarios in which all our experiences are misleading.

Best argument

The argument is short. If you know you have hands, you know you are not a handless brain in a vat. You do not know you are not a brain in a vat, since everything would seem the same if you were. So you do not know you have hands. Each premise seems true, and the logic is valid.

Strongest objection

The conclusion is unbelievable, and no one can actually live by it. Many philosophers think we have more reason to believe we know we have hands than to believe any premise of a clever argument to the contrary.

Defended by

Sextus Empiricus, Peter Unger, Barry Stroud

Common sense (Moore)

We know many things about the world, and any argument concluding otherwise must have a false premise.

Best argument

G. E. Moore famously held up his hands before an audience and declared that here was one hand, and here was another. He argued that he was more certain of this than of any premise in a skeptical argument. So rather than accept the conclusion, we should reverse the argument: I know I have hands, so I know I am not a brain in a vat.

Strongest objection

To many this seems to beg the question. The skeptic is asking how you know you have hands, and Moore simply repeats that he does.

The reply

Moore's defenders argue that not every piece of knowledge needs a further argument behind it. Some things are known directly, and it is the skeptic who owes us a reason to doubt them.

Defended by

G. E. Moore, Thomas Reid

Contextualism

Whether you 'know' something depends on the standards in play. In everyday contexts, you know you have hands; in a philosophy seminar, the standards rise and you do not.

Best argument

The word 'know' is context-sensitive, like 'tall' or 'flat'. When a skeptic raises the possibility of a demon, they shift the conversation to extremely demanding standards. Both the skeptic and the ordinary person can be right, each in their own context.

Strongest objection

It seems to change the subject. The skeptic wants to know whether we really know, and being told that 'know' means different things in different conversations does not answer that.

Defended by

Keith DeRose, David Lewis, Stewart Cohen

Deny closure

You can know you have hands without knowing you are not a brain in a vat. Knowledge is not closed under known implication.

Best argument

Fred Dretske and Robert Nozick argued that knowledge requires 'tracking' the truth: if it were false, you would not believe it. Your belief that you have hands tracks the truth (if you had no hands, you would notice). Your belief that you are not a brain in a vat does not. So you know the first without knowing the second.

Strongest objection

It produces 'abominable conjunctions', as Keith DeRose put it: 'I know I have hands, but I do not know I am not a handless brain in a vat.' That sounds incoherent.

Defended by

Fred Dretske, Robert Nozick

Where you stand

Can you know the world outside your mind exists?

Choose one to see how people answer.

For the full background, variations and open questions, read the entry: How do you know anything at all?.