ABSTRACT
Tweedledum and Tweedledee were alike in all respects. There was nothing that could be truthfully said of Tweedledum that could not be truthfully said of Tweedledee. Tweedledum was fat: so was Tweedledee. Tweedledee wore a schoolboy cap: so did Tweedledum. Tweedledum was quarrelsome, but timid in the face of unorthodox ornithological monstrosities: exactly the same could be said of Tweedledee. In some cases the concept qualitatively identical but numerically distinct is clearly inapplicable. With numbers, it makes no sense to think of two different numbers being alike in all respects. The elements in the periodic table are paradigm examples of secondary substances: they cannot be qualitatively identical but numerically distinct, and, by the same token, they cannot change. If the fundamental furniture of the world consists of Platonic forms or secondary substances of which it makes no sense to talk of two being qualitatively identical but numerically distinct, then it must be impossible also to talk of things changing.
