Full Idea
Functions of one argument are concepts; functions of two arguments are relations.
Gist of Idea
Relations are functions with two arguments
Source
Gottlob Frege (Function and Concept [1891], p.39)
Book Reference
Frege,Gottlob: 'Translations from the Writings of Gottlob Frege', ed/tr. Geach,P/Black,M [Blackwell 1980], p.39
A Reaction
Nowadays we would say 'two or more'. Another interesting move in the aim of analytic philosophy to reduce the puzzling features of the world to mathematical logic. There is, of course, rather more to some relations than being two-argument functions.