Thought experiments i

["james cunningham" <pdjamesc@hotmail.com>]

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I think that you are rather oversimplifying the role that thought 
experiments can play, but you do raise an interesting point.

If the question you are raising is to what extent (if any) conceivability is 
a guide to metaphysical possibility (and to what extent inconceivability is 
a guide to impossibility), then you are opening a vast can of worms. The 
consensus seems to be ?not a great deal?. I wrote something on this about a 
year ago, which I'll paste bellow. It is mostly about Yablo, and I'm sure it 
will make you sob with boredom. No footnotes I'm afraid, and the logic 
symbols have been lost somewhere. Ah, well.


Is Conceivability a Guide to Possibility?

Parte Ye Firste

The idea that conceivability is a guide to possibility, and the slightly 
different but closely related notion that inconceivability is a guide to 
impossibility, has a good deal of intuitive support. In everyday use we use 
the words conceivable and inconceivable interchangeably with possible and 
impossible. However, as is the case with so much else in philosophy, 
ordinary usage is not an adequate guide. Hume, in the Treatise on Human 
Nature, suggests that it is:

?An established maxim in metaphysics, that whatever the mind clearly 
conceives, includes the idea of possible existence, or in other words, that 
nothing we imagine is absolutely impossible.?i

But if this is taken to mean that conceivability is sufficient to prove 
possibility I think that there are few who would find this plausible. It 
seems conceivable that gold is a compound, yet, if we accept Kripke and 
Putnam?s view, it is necessarily false. Never the less there is an almost 
universal tendency to rely on intuition during discourse about modality. The 
instruction: ?imagine a possible world? is exceedingly common, and very 
difficult to employ without some version of the thesis that conceivability 
gives a guide to possibility.

Steven Yablo, in his paper Is Conceivability a Guide to Possibility?ii
makes the assumption that conceiving, involves the appearance of 
possibility, or at least that there is a sense in which the appearance of 
possibility is necessary (although not sufficient) for conceivability. Yablo 
claims that this sense conceivability does provide a guide to possibility.

Believability

Before examining what conceivability is, Yablo first gives some examples of 
what it is not. The first notion discussed is that for a proposition to be 
conceivable means that it is true for all you know. For p to be conceivable 
to a is equivalent to saying that a does not know that not-p. Yablo calls 
this believability. If we are to take believability as conceivability it is 
difficult to see how it could be that the conceivability of p could provide 
a reliable indication the possibility of p. Yablo argues that if this were 
the case then my lack of evidence for the truth of p would be evidence for 
the possibility of p. While I think the underlying point Yablo is making is 
sound (more of which anon) I think this characterisation is somewhat unfair. 
Rather I think that a lack of evidence that not-p would provide evidence for 
the possibility of p. Never the less Yablo is correct in claiming that 
believability will be very unreliable evidence for possibility. The problem 
arises from the confusion of epistemic possibility with metaphysical 
possibility. Although in ordinary conversation, as hinted at above, 
conceivability and possibility, are both often used to mean believability 
the three are not identical in any philosophical sense. At this time I find 
it believable that all natural numbers are the sum of two primes. Equally I 
find the opposite believable. Whichever one is actually the case it will be 
necessarily so. We are left with the solution that believability and 
conceivability are quite different things. It is straightforward to arrive 
at an example where a proposition is conceivable but not believable. Yablo 
gives the example that it is conceivable for him never to have existed and 
yet not believable (to him) for him never to have existed. An example of 
believability without conceivability is tricky because if p is believable it 
may be the case that p is true in the actual world, and if so it will be 
possible. Never the less there is no reason that a proposition may not be 
believable whilst being neither or conceivable and inconceivable.

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