Biologically, life, as contrasted with death or with nonliving objects, is an evident fact but difficult to characterize precisely. Living organisms are self-maintaining systems; they grow and are irritable in response to stimuli. They resist dying. They reproduce. The developing embryo is especially impressive. Organisms post a defended, semipermeable boundary between themselves and the outside world; they assimilate environmental materials to their own needs. They can be healthy or diseased. Some accounts claim that the minimal form of autonomy necessary and sufficient for characterizing biological life is what is termed autopoiesis, literally self-making. Some defense of a "self" (a somatic self, having to do with the body, rather than a psychological self) is thus required.

Living organisms gain and maintain internal order against the disordering tendencies of external nature. They keep recomposing themselves, while...