Full Idea
The Axiom of Reducibility asserted that to every non-elementary function there is an equivalent elementary function [note: two functions are equivalent when the same arguments render them both true or both false].
Gist of Idea
Reducibility: to every non-elementary function there is an equivalent elementary function
Source
Frank P. Ramsey (The Foundations of Mathematics [1925], §2)
Book Reference
Ramsey,Frank: 'Philosophical Papers', ed/tr. Mellor,D.H. [CUP 1990], p.191
A Reaction
Ramsey in the business of showing that this axiom from Russell and Whitehead is not needed. He says that the axiom seems to be needed for induction and for Dedekind cuts. Since the cuts rest on it, and it is weak, Ramsey says it must go.