I offer a novel interpretation of Aristotle's psychology and notion of rationality, which draws the line between animal and specifically human cognition. Aristotle distinguishes belief (doxa), a form of rational cognition, from imagining (phantasia), which is shared with non-rational animals. We are, he says, “immediately affected” by beliefs, but respond to imagining “as if we were looking at a picture.” Aristotle's argument has been misunderstood; my interpretation explains and motivates it. Rationality includes a filter that interrupts the pathways between cognition and behavior. This prevents the subject from responding to certain representations. Stress and damage compromise the filter, making the subject respond indiscriminately, as non-rational animals do. Beliefs are representations that have made it past the filter, which is why they can “affect [us] immediately.” Aristotle's claims express ceteris paribus generalizations, subject to exceptions. No list of provisos could turn them into non-vacuous universal claims, but this does not rob them of their explanatory power. Aristotle's cognitive science resolves a tension we grapple with today: it accounts for the specialness of human action and thinking within a strictly naturalistic framework. The theory is striking in its insight and explanatory power, instructive in its methodological shortcomings.