Introduction to Mathematical Philosophy

by Bertrand Russell

is human" or "

is mortal." Thus extensional functions of a function

may, for practical purposes, be regarded as functions of the class determined by

, while intensional functions cannot be so regarded.

It is to be observed that all the specific functions of functions that we have occasion to introduce in mathematical logic are extensional. Thus, for example, the two fundamental functions of functions are: "

is always true" and "