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Only a living entity can have goals or can originate them. And it is only a living organism that has the capacity for self-generated, goal-directed action. On the physical level, the functions of all living organisms, from the simplest to the most complex — from the nutritive function in the single cell of an amoeba to the blood circulation in the body of a man — are actions generated by the organism itself and directed to a single goal: the maintenance of the organism’s life.

An organism’s life depends on two factors: the material or fuel which it needs from the outside, from its physical background, and the action of its own body, the action of using that fuel properly. What standard determines what is proper in this context? The standard is the organism’s life, or: that which is required for the organism’s survival."

Ayn Rand, The Virtue of Selfishness, 16