Associationism is one of the oldest, and, in some form or another, most widely held theories of thought. Associationism has been the engine behind empiricism for centuries, from the British Empiricists through the Behaviorists and modern day Connectionists. Nevertheless, “associationism” does not refer to one particular theory of cognition per se, but rather a constellation of related though separable theses. What ties these theses together is a commitment to a certain arationality of thought: a creature’s mental states are associated because of some facts about its causal history, and having these mental states associated entails that bringing one of a pair of associates to mind will, ceteris paribus, ensure that the other also becomes activated.