To distinguish the extensional concept of functions-as-sets from the non-extensional concept of functions-as-rules, the latter is often referred to as an ‘intensional’ function concept, in part because of the ostensibly intensional concept of a rule involved. This terminology is particularly predominant in the community of mathematical logicians and philosophers of mathematics working on the foundations of mathematics. But from the perspective of the philosophy of language, the terminology can be somewhat misleading, since in this context, the extensional-intensional distinction has a slightly different meaning.