 The first “sortal logic” was developed in Stevenson (1975). Stevenson develops an account in which he says “We use ideas of Geach, Wallace and Wiggins, although we depart from each in certain respects” (1975: 186). Although the formalism follows Wallace in some respects, the main ideas seem to be those of Wiggins rather than Geach. Specifically, his theorem 3.4.5 affirms that for any sortals F, G, if b and c are the same F, and b is a G, then b and c are the same G, which is a denial of Geach’s relative identity thesis.