Full Idea
Von Neumann suggested that functions be pressed into service to replace sets.
Gist of Idea
Von Neumann wanted mathematical functions to replace sets
Source
report of John von Neumann (works [1935]) by José A. Benardete - Metaphysics: the logical approach Ch.23
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.188