Quidditism is the property-theoretic analogue of haecceitism. (Armstrong (1989: 59) seems to coin the term ‘quidditism.’) But, while distinctively haecceitistic possibilities involve individuals being “swapped” or “replaced” without any qualitative difference, distinctively quidditistic possibilities involve properties swapping or replacing one another in the world’s causal-nomic structure. If, for example, there is a maximal possibility, otherwise indiscernible from actuality, where mass and charge swap their respective causal-nomic roles (e.g., where objects resist acceleration in virtue of their charge), this maximal possibility would differ quidditistically from actuality. Similarly, if there is a maximal possibility according to which all actual causal-nomic facts are the same except that some actually uninstantiated property, schmass, occupies the causal and nomic role of mass, this maximal possibility would also differ quidditistically from actuality. (Typically, quidditism is taken as a thesis about fundamental or “sparse” properties like mass and charge rather than as a thesis about all properties whatsoever.)