“Reason is man’s tool of survival. From the simplest necessity to the highest abstraction, summarizes ‘The Fountainhead‘, ‘from the wheel to the skyscraper, everything we are and everything we have comes from a single attribute of man—the function of his reasoning mind.’”
— Leonard Peikoff, “O.P.A.R.”, page 198.
(A pre-historic mammoth hunt showing a great number of men using arrows, spears and knifes to overcome one single mammoth. Could they do it by instinct? No. Perhaps they could when they were apes. But the moment they stopped being apes, their survival depended on their minds. Man is the rational animal, “because man is the organism that survives by its use.”)
A system of thought must provide a philosophical understanding of the nature of man. The metaphysical nature of man, as Ayn Rand put it, is what links the broad abstract principles at the base of any one system to the practical decisions at its apex. If you don’t know what you are, you won’t be able to decide correctly what to do in any given situation. For example, if you are a cell of a larger whole, whether of Society or of God, you will behave according to the dictates of one of them; if you are “just” an individual, you will act as one.