Nozick and world tracking

Nozick's response to Gettier's objection to the justified true belief analysis is to replace it with a notion of truth tracking in which no mention is made of justification. The idea is that one has knowledge of some fact if one's belief is responsive to or tracks that fact. Roughly, one would only believe that p if p were the case. If that is the nature of the link then it seems that there is no space for (knowledge undermining) luck in the relation between believing that p and p. Nozick cashes this intuition out using four conditions.

S knows that p iff

p is true;

S believes that p;

S would not believe p were 'p' not true;

S would believe p were 'p' true.

So these conditions are a formal unpacking of the idea of tracking. But they themselves are further unpacked by talk of nearby possible worlds to explain the third and fourth condition. The subjunctive conditional (in the third and fourth claim) p → q is not the same as the conditional ‘if p then q’ (which says it is never true that p is true and q false.) Nor is it ‘it is logically impossible that p and not q’. The subjunctive conditional doesn’t say anything about lots of the cases of p being true.

Rather: in all the possible worlds similar to ours where p is true, q is also true. So the idea is that we examine those worlds in which p holds true closest to the actual world and see if q holds in all of those. We should ignore p worlds which are quite unlike ours.

One consequence of this is that it turns out that we can know that we are in Preston even though we do not know we are not brains in a vat and even though we know that if we are brains in a vat we are not in Preston. Nozick bites on the bullet and simply denies what is called 'epistemic closure': If person S knows p, and p entails q, then S knows q.

In denying this he follows Fred Dretske who gives the following example:

You take your son to the zoo, see several zebras, and, when questioned by your son, tell him they are zebras. Do you know they are zebras? Well, most of us would have little hesitation in saying that we did know this. We know what zebras look like, and, besides, this is the city zoo and the animals are in a pen clearly marked "Zebras.” Yet, something's being a zebra implies that it is not a mule and, in particular, not a mule cleverly disguised by the zoo authorities to look like a zebra. Do you know that these animals are not mules cleverly disguised by the zoo authorities to look like zebras?

If you are tempted to say "Yes" to this question, think a moment about what reasons you have, what evidence you can produce in favour of this claim. The evidence you had for thinking them zebras has been effectively neutralized, since it does not count toward their not being mules cleverly disguised to look like zebras. Have you checked with the zoo authorities? Did you examine the animals closely enough to detect such a fraud?

You might do this, of course, but in most cases you do nothing of the kind. You have some general uniformities on which you rely, regularities to which you give expression by such remarks as, "That isn't very likely" or "Why should the zoo authorities do that?" Granted, the hypothesis (if we may call it that) is not very plausible, given what we know about people and zoos. But the question here is not whether this alternative is plausible, not whether it is more or less plausible than that there are real zebras in the pen, but whether you know that this alternative hypothesis is false. I don't think you do.

In this I agree with the skeptic. I part company with the skeptic only when he concludes from this that, therefore, you do not know that the animals in the pen are zebras. I part with him be- cause I reject the principle he uses in reaching this conclusion-the principle that if you do not know that Q is true, when it is known that P entails Q, then you do not know that P is true. [Dretske 1970: 1016]

Reading

- Nozick, R. (2010) 'Knowledge and scepticism' Sosa et al (eds) (2010) Epistemology: an anthology, Oxford Blackwell. Read as far as p269 'sins of credulity'. (A crude pdf of the text can be found here.)