It is to be observed that what is fundamental is not particular propositional functions, but the class-concept *propositional function*. A propositional function is the class of all propositions which arise from the variation of a single term, but this is not to be considered as a definition, for reasons explained in the preceding chapter.(§ 91 ¶ 1)

The Principles of Mathematics was written by Bertrand Russell, and published in in 1903. It is now available in the Public Domain.