An excellent illustration of this reductive process is provided by Leonard Peikoff in Objectivism: The Philosophy of Ayn Rand (New York: Meridian, 1993), pp. 134-5 (more information). Peikoff traces the meaning of the complex concept "friend" through abstract concepts such as "esteem" and "value," then through earlier concepts such as "purpose" and "choice," and finally to the first-level concept (closest to the perceptual level) "man."