Full Idea
One of the glories of Von Neumann's theory of numbers is to define α < β to mean that α ∈ β.
Gist of Idea
Von Neumann defines α<β as α∈β
Source
William D. Hart (The Evolution of Logic [2010], 3)
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.74