Plato’s student, Aristotle, had a different notion in which living things had an appropriate form, material, and goal-directedness (De Anima, 412a1–416b). Aristotle held life to be a form of self-motion, perpetuation, or self-alteration (Byers 2006). For Aristotle, the capacity to resist internal and external perturbations was the essential distinction between living beings and non-living objects. Other features were accidental. This quest for demarcating the essential from the accidental for life has persisted to this day in searches for theoretical definitions of life as well as in attacks against those not interested in such definitions.