Intuition and the Substitution Argument

Superman has a bad allergy to kryptonite. And since Superman is Clark Kent, it follows that Clark Kent is allergic to kryptonite. Which, indeed, he is. If you were to take some kryponite into the offices of the Daily Planet, Clark would soon be feeling quite poorly. Of course, not many people know that Clark is allergic to kryptonite. In particular, Superman’s nemesis Lex Luthor does not know that Clark is allergic to kryptonite, which is why he does not sneak into the newsroom and hide some kryptonite in Clark’s desk. But of course, Luthor knows perfectly well that Superman is allergic to kryptonite, and he frequently tries to expose him to it. That, at least, is the sort of thing one would usually hear people say. Familiarly, however, taking such remarks at face value generates problems. The most immediate of these is that it requires us to restrict the logical principle known as “substitution of identicals”: