Emergence is the rise of a system that cannot be predicted or explained from antecedent conditions. George Henry Lewes, the 19th-century English philosopher of science, distinguished between resultants and emergents--phenomena that are predictable from their constituent parts and those that are not (e.g., a physical mixture of sand and talcum powder as contrasted with a chemical compound such as salt, which looks nothing like sodium or chlorine). The evolutionary account of life is a continuous history marked by stages at which fundamentally new forms have appeared: (1) the origin of life; (2) the origin of nucleus-bearing protozoa; (3) the origin of sexually reproducing forms, with an individual destiny lacking in cells that reproduce by fission; (4) the rise of sentient animals, with nervous systems and protobrains; and (5) the appearance of cogitative animals, namely humans. Each of these new modes of life, though grounded in the physicochemical and biochemical conditions of the previous and simpler stage, is intelligible only in terms of its own ordering principle. These are thus cases of emergence.

Excerpt from the Encyclopedia Britannica without permission.