What are and Why Study Later Medieval Theories of Animal Rationality?Contemporary theories of animal rationality concern a relatively wide range of issues. They range from inferential and linguistic capacities to social and meta-cognition.1 What is common to most of them is that they are interested in process rather than behavioural rationality.2 Generally speaking, these are different ways of telling whether or not a being is rational. When we focus solely on its behaviour and say that it is rational, since, for instance, it chooses the right means to achieve a certain end (and so 'maximises utility' , as some people put it) we ascribe behavioural rationality to that being. According to this concept of rationality, almost any kind of being could count as rational in one way or another. Not only people can be rational but also institutions, machines, or genes as long as they behave in a way that qualifies as rational.3 What matters is simply whether or not a certain behaviour can be described as rational. If this is the case, then the being exhibiting this behaviour can be called rational.However, this type of rationality is much more interesting and relevant for economists, for example, than it is for psychologists. The latter are interested rather in the question whether the behaviour we observe is based on rational processes. To put it slightly differently, they wonder how a certain kind of behaviour is brought about. For instance, they examine whether a certain kind of behaviour that can be described as rational from the outside is actually based on a rational cognitive process such as reasoning. To be clear, this does not mean that psychologists or anyone else interested in process rationality work with a better or more appropriate concept of rationality than economists and people interested in behavioural rationality. They simply look at rationality from a different angle, and when they identify rationality they do this at a different level.In the Middle Ages, one can also find this distinction between behavioural and process rationality -not literally, of course, but systematically. A passage that neatly illustrates this is found in Thomas Aquinas' Summa theologiae.