Full Idea
A truly infinite quantity (for example, the length of a straight line, unbounded in either direction) does not by any means need to be a variable.
Gist of Idea
A truly infinite quantity does not need to be a variable
Source
Bernard Bolzano (Paradoxes of the Infinite [1846]), quoted by Brian Clegg - Infinity: Quest to Think the Unthinkable §10
Book Reference
Clegg,Brian: 'Infinity' [Robinson 2003], p.131
A Reaction
This is an important idea, followed up by Cantor, which relegated to the sidelines the view of infinity as simply something that could increase without limit. Personally I like the old view, but there is something mathematically stable about infinity.