But an extensional theory is, of course, not thereby free of problems. Consider, for example, the expression “unicorn” and “centaur”, which have the same extension (namely the null-extension) but differ in meaning. Hence, whereas sameness of extension is a necessary condition for sameness of meaning, sameness of extension does not seem to be sufficient for sameness of meaning. Goodman proposes an extensional fix to this problem that gives necessary and sufficient conditions for sameness of meaning. He observes that although “unicorn” and “centaur” have the same extension, simply because of the trivial fact that they denote nothing, “centaur-picture” and “unicorn-picture” do have different extensions. Clearly, not all centaur-pictures are unicorn-pictures and vice versa. Thus the flight to compounds makes an extensional criterion possible: