If ad hoc sortals were permitted, any putative contradiction generated by relative identity could be avoided by deeming one sortal a phase sortal and the other an ultimate sortal.

More importantly, that there are no ad hoc sortals is essential to the significance of the formal proof that there is no such thing as relative identity. In logic and in the application of Leibniz’s Law, ‘a is F’ is normally equivalent to ‘a was, is or will be F’, otherwise the Law would not apply to such accidental properties as ‘is brown’. This is also the way phase sortals work: a human being is, was, or will be a baby. The argument against relative identity works by arguing that, if one allo