"Biology has traditionally started at the top, viewing a living organism as a complex bio-chemical machine, and worked analytically downwards from there-through organs, tissues, cells, organelles, membranes, and finally molecules-in the pursuit of the mechanisms of life. Artificial Life starts at the bottom, viewing an organism as a large population of simple machines, and works upwards synthetically from there- constructing large aggregates of simple rule governed objects which interact with one another nonlinearly in the support of life-like global dynamics."
Chris Langton