 Two millennia after Aristotle, Descartes' mechanistic philosophy introduced the idea of a reflex to explain the behavior of nonhuman animals.  Although his conception of animals treated them as reflex-driven machines, with no intellectual capacities, it is important to recognize that he took mechanistic explanation to be perfectly adequate for explaining sensation and perception — aspects of animal behavior that are nowadays often associated with consciousness. He drew the line only at rational thought and understanding. Given the Aristotelian division between instinct and reason and the Cartesian distinction between mechanical reflex and rational thought, it's tempting to map the one distinction onto the other. Nevertheless, it may be a mistake to assimilate the two. First, a number of authors before and after Darwin have believed that conscious experience can accompany instinctive and reflexive actions. Second, the dependence of phenomenal consciousness on rational, self-reflective thought is a particularly strong and contentious claim (although it has current defenders, discussed below).