A concept is a mental integration of two or more units which are isolated by a process of abstraction and united by a specific definition. By organizing his perceptual material into concepts, and his concepts into wider and still wider concepts, man is able to grasp and retain, to identify and integrate an unlimited amount of knowledge, a knowledge extending beyond the immediate concretes of any given, immediate moment.

- Ayn Rand. The Romantic Manifesto (1969).