#2d semantics

Are there cases where we can argue from conceivability to possibility? Yes and no.

(1) Something is conceivable if we cannot rule it out a priori.

(2) Something is possible if it doesn't contradict a necessary truth.

(3) We can argue from conceivability to possibility if and only if a concept is semantically stable, i.e. has the same intension across a 2-dimensional semantic framework.

Cases where we can't make the move: The case of water. It's conceivable that 'water=XYZ' is true. Such an identity would only be discoverable a posteriori, and therefore we cannot rule it out on a priori grounds. In the actual world, 'water=H2O' expresses a necessary truth. So even though it is conceivable that 'water=XYZ' is true, it is not possible, because 'water=XYZ' contradicts a necessary truth. On the primary intention, corresponding generally to the Fregean sense, Water= odorless, colorless, liquid, found in lakes etc. The secondary intension, corresponding generally to the Kripkean meaning, water=H20. Therefore our concept of water is unstable, i.e. the primary intension differs from the secondary intension. We cannot infer the possibility that 'water=XYZ' is true from the fact that it is conceivable.

Cases where we can make the move: The case of Zombies. It is conceivable that 'pain≠c-fibers firing' is true (where the 'c-fibers firing' is a placeholder for a complete physical description of some brain state). We cannot rule out 'pain≠c-fibers firing' on a priori grounds. In the actual world, and in all possible worlds, pain is ontologically subjective, so 'pain≠c-fibers firing' does not contradict a necessary truth. This becomes more apparent when we check the semantic stability of pain across the 2D semantic framework. The primary and secondary intension of pain is the same; on both intensions pain is just what it is like to feel pain. There is no strong dissociation between the appearance and reality of pain, as there was in the case of water. Pain=the quality of pain, localized shooting, unpleasant sensations, etc in both the case of primary intension and secondary intension. So if we can conceive of pain existing without a corresponding brain state, we thereby admit the possibility of pain existing without a corresponding brain state. 

It is possible to object to certain brands of materialism on the basis that pain may exist without some corresponding brain state. See: Chalmers' argument against materialism