Review: Smart/Place: Sensation-Brain identity statements are synthetic a posteriori. As a result, they are contingent—it is possible for them to be false.

Kripke's revelation

In his book, Naming and Necessity, Kripke argues quite convincingly that some (true) a posteriori identity statements are necessary…it is impossible for them to be false! According to Kripke, an a posteriori identity statement is necessary if both the terms involved are rigid designators. (A term is a “rigid designator” if it refers to the same object in all possible worlds.)

Question: Is "Pain" a rigid designator?
Smart's answer: NO. "Pain" picks out its referent by a "topic-neutral" property. Lots of different things could have this property. Hence, "pain" can refer to different things in different possible worlds.
Kripke's answer: YES. "Pain" picks out its referent by an essential property (a property the referent couldn't fail to have)—the feeling of painfulness. Hence, "pain" is a rigid designator--it refers to the same thing in all possible worlds. If Kripke is right about this, then "pain = C-fiber firing" is a necessary identity statement. But if it's necessary, why does it seem like it is contingent?
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Why do "empirical" identity statements seem contingent?
(Note: an "empirical" identity statement is an identity statement that is both a posteriori and necessary.)

As Kripke points out in Naming and Necessity, other empirical identity statements (like “lightning = electrical discharge”) can seem contingent even though they are necessary. He offers the following model to explain this:

Kripke's model: "Lightning = electrical discharge" seems contingent because the referent of "lightning" was fixed by the following property—that flash of light seen during stormy nights. This identity statement seems contingent because we can imagine a world where people see similar flashes of light that are not the result of electrical discharge. (If we are careful, however, we will realize that this imagined world is merely a world where something that looks like lightning fails to be electrical discharge, not a world where lightning is not electrical discharge.)

Although this model explains why "lightning = electrical discharge" seems contingent, it does not explain why "pain = C-fiber firing" seems contingent. Something can look like lightning but fail to actually be lightning — that’s why “lightning = electrical discharge” seems contingent even though its necessary. Something cannot feel like pain but fail to be pain, however, because if something feels like pain, it’s pain!! Hence, Kripke argues, pain cannot be C-fiber firing.

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