Full Idea
I take the notion of the variable as fundamental.
Clarification
A variable is something like x in algebra, which has a value which isn't fixed
Gist of Idea
The idea of a variable is fundamental
Source
Bertrand Russell (On Denoting [1905], p.42)
Book Reference
Russell,Bertrand: 'Logic and Knowledge', ed/tr. Marsh,Robert Charles [Routledge 1956], p.42
A Reaction
A key idea of twentieth century philosophy, derived from Frege and handed on to Quine. A universal term, such as 'horse', is a variable, for which any particular horse can be its value. You can calculate using x, and generalise about horses.