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Thought experiments iii
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Is Conceivability a Guide to Possibility?
Parte Ye Thirde
The a Posteriori Objection
Another possible problem for the defender of modal intuition may arise from
Kripke and Putnam?s notion of a posteriori necessary truths, e.g. that cats
are animals or the morning star is the evening star. If this is true then
there are also propositions that are impossible but are only knowable as
such a posteriori. If we are to accept this then the following argument may
be constructed:
2) Whenever p is a posteriori false, I find it conceivable whether it is
possible or not.
3) Often propositions, which are a posteriori false, are impossible.
Therefore
4) A posteriori falsehoods are often found conceivable despite their
impossibility.i
The conclusion of this argument, if true, leads to some uncomfortable
problems. I can conceive of myself closing a window (and not actually doing
so), yet it may be impossible. According to Putnam (although not Kripkeii)
to conceive a proposition is to imagine acquiring evidence that justifies
you in believing it. How does this apply to conceiving impossibilities? To
conceive that p is true is to imagine oneself having a justified true belief
that p. In this sense of conceivability an a posteriori falsehood is quite
conceivable, but Hume?s maxim (as interpreted by Yablo) claims that only a
sense of conceivability that portrays p as possible is of evidence of modal
status.
Conceivability
So what according to Yablo is conceivability? a finds p conceivable if a can
imagine a situation of which he can truly believe that p. To imagine a
situation may be thought of as imagining a more or less incomplete possible
world. So conceiving that p is imagining a world in which p is a true
description.
(CON) I can imagine a world that I take to verify p.
Similarly
(INC) I cannot imagine any world that I don?t take to falsify p.iii
This explanation produces some interesting points. As Yablo says we cannot
imagine a tiger with round-square stripes or a pile containing more salt
than sodium chloride. There are two explanations for this:
5) We cannot imagine a situation unless it already appears that it could
exist.
6) To imagine a situation is thereby to enjoy the appearance that it could
exist.iv
5) is problematic because it states that we could never arrive at the view
?a is possible? by imagining it, which is a usual way of coming to regard
things as possible. Adopt 6) then. This also gives some explanation to the
earlier maxim that conceiving involves the appearance of possibility.
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