ABSTRACT
This chapter is concerned with the in the plural: the inhabitants of London, the sons of rich men, and so on. It is concerned with classes. The theory of classes is less complete than the theory of descriptions, and there are reasons for regarding the definition of classes that will be suggested as not finally satisfactory. Some of the things that may be said about a function may be regarded as said about the class defined by the function, whereas others cannot. The axiom is a generalised form of Leibniz’s identity of indiscernibles. Leibniz assumed, as a logical principle, that two different subjects must differ as to predicates. Leibniz’s assumption is a much stricter and narrower one than ours.
