Von Neumann proposed a foundation for mathematics in which "function" was a primitive notion. A set is a special kind of function. (Perhaps a function taking only values 0 and 1, I no longer remember the details. Of course 0 and 1 are themselves certain functions.) But in short order more conventional mathematicians re-cast this into an equivalent system with sets as primitive. But to do that, "proper classes" had to be allowed, as well as sets. http://en.wikipedia.org/wiki/Von_Neumann%E2%80%93Bernays%E2%80%93G%C3%B6del_set_theory
|
1 | [made Community Wiki] | ||
|
|
||||
