 By carrying out this analysis in the special setting of an algebra of predicates (or equivalently, in an algebra of classes) Jevons played an important role in the development of modern equational logic. As mentioned earlier, Boole gave inadequate sets of equational axioms for his system, originally starting with the two laws due to Gregory plus his idempotent law; these were accompanied by De Morgan’s inference rule that one could carry out the same operation (Boole’s fundamental operations in his algebra of logic were addition, subtraction and multiplication) on equals and obtain equals. Boole then switched to the simple and powerful (but unexplained) Rule of 0 and 1.