 One motivation stems from Leibniz’s observation that a conjunctive treatment of the genus-species relation does not fix a direction of specification, since one can as easily take the differentia to be the genus, as vice versa (see Nouveaux Essais, III, iii, 10, Langley’s translation, 313). Johnson (1924) interprets Descartes and Spinoza as attempting to ensure asymmetry by means of attributes and modes (see Descartes, Principles of Philosophy I.53; Spinoza, Ethics 1p8s1 and Ep. 50), which Johnson suggests are “almost equivalent” to the notions of determinable and determinate: 