Rigid Designators
Rigid designators are names which pick out the same object in every possible world in which that object exists. Whether RDs exist depends on whether things have essences. If they do, then in any possible world a thing is either identical to some corresponding thing in the actual world or not. Otherwise, things in possible worlds or counterfactual situations resemble the original object in the actual world more or less; they are mere “counterparts.” Even if by “Cicero” we mean “the man who wrote such and such works in Latin,” the description is of an accident, whereas the name refers to an essence. Therefore, in some possible world there may be a person called Cicero who, however, failed to write anything. An interesting question, discussed by Kripke, is whether Cicero is necessarily identical to Tully, whom we can define, again, by an accident of having denounced Cataline, if it so happens that in the actual world Cicero and Tully are one and the same person. Kripke argues that yes, both Cicero and Tully are rigid designators and are therefore identical in every possible world. Now if the only way we can identify Cicero is by the works he wrote, and in some possible world he did not write these works, then how can we possibly find him in that world? Thankfully, this is an epistemological rather than metaphysical question. If we knew Cicero in exhausting detail, e.g., as God knows him, then we could separate his essence from his accidents and find out quite easily whether any given possible world does or does not contain his essence.
What about the identity of heat and the motion of molecules? It is an a posteriori scientific claim. How can it possibly be necessary? Kripke imagines Martians who had a different neural structure from ourselves and felt heat when exposed to cold and felt cold when exposed to heat. “But still,” he writes, “heat would be heat, and cold would be cold.” It seems to me that he claims that we learn what heat is by feeling it. Then science tells us what heat is physically. Again, heat is contingently identified via “the fact that there happen to be creatures on this planet — (namely, ourselves) who are sensitive to it in a certain way.” But that’s not the essence of heat. The essence of heat is fast molecular motion. Hence “heat” and “fast molecular motion” are rigid designators, and so identity between them is necessary. Kripke’s analysis is misleading. Heat is a sensation, an experience that a creature feels; it is not “really” “the motion of molecules,” no more than an experience of, say, reaching a decision is “really” an excitation of C-fibers in the brain. He might want to define heat as “the sensation that would be felt by humans when exposed to fast molecular motion, if they lived on earth.” But then he could only argue that molecular motion causes or is correlated with the sensation of heat. But the Martians wouldn’t think that. Hence the identity is not necessary.
The pattern then is as follows. Let A = B, and let an accidental aspect of A be described by one contingent statement α, and of B, by another contingent statement β. But if both A and B referred to the same essence, then that essence must persist in every possible world regardless of what happens to α and β, and therefore A is identical with B necessarily.