Through the process of nutrition, the raw materials the organism needs are brought into its system; through
respiration (oxidation), energy is then extracted from these raw materials; a part of this energy is then used in the
process of synthesis which transforms the raw materials into structural components of living matter. The remaining
energy, together with all the structural components, makes possible the continuation of the organism's self-
maintaining activity. Metabolism characterizes all living species.
But now consider an example of the wider principle involved, that is peculiar to man: the activity of harnessing a
waterfall in order to obtain the electric energy needed to power a factory engaged in the manufacture of farm
equipment or clothing or automobiles or drugs. Here, the action is external rather than internal, behavioral rather
than metabolic; but the basic principle of life remains the same.
The existence of life is conditional; an organism always faces the possibility of death. Its survival depends on the
fulfillment of certain conditions. It must generate the biologically appropriate course of action. What course of
action is appropriate, is determined by the nature of the particular organism. Different species survive in different
ways.
An organism maintains itself by exercising its capacities in order to satisfy its needs. The actions possible to and
characteristic of a given species, are to be understood in terms of its specific needs and capacities. These constitute
its basic behavioral context.
"Need" and "capacity" are used here in their fundamental, metaphysical sense (by "metaphysical," I mean:
pertaining to the nature of things); in this context, "need" and ''capacity" refer to what which is innate and universal
to the species, not to that which is acquired and peculiar to the individual.
An organism's needs are those things which the organism, by its nature, requires for its life and well-being—i.e., for
its efficacious continuation of the life-process. An organism's capacities are its inherent potentialities for action.
The concept of needs and capacities is fundamental to biology and psychology alike. Biology is concerned with the
needs and capacities of living organisms qua physical entities. Psychology is concerned with the needs and
capacities of living organisms qua conscious entities.