Pure logic was devised as an aid for practical uses in argumentation. For MacColl this practical character is due to its abstract nature, which in turn makes generalisations possible. Since pure logic is abstract, its categorical symbols do not stand for numbers or for classes of objects or temporal states, but more generally for statements or propositions. Boole’s logic, in which propositional operations are translated into class operations, is basically not general enough and should be replaced by a Calculus of Equivalent Statements in which propositional connectives occur instead of Boole’s equations.2