Pieces of behaviour, beliefs, arguments, policies, and other exercises of the human mind may all be described as rational. To accept something as rational is to accept it as making sense, as appropriate, or required, or in accordance with some acknowledged goal, such as aiming at truth or aiming at the good. Although it is frequently thought that it is the ability to reason that sets human beings apart from other animals, there is less consensus over the nature of this ability: whether it requires language, for example (see also animal thought). Some philosophers (Plato, Aristotle) have found the exercise of reason to be a large part of the highest good for human beings. Others (Kant, Hegel) find it to be the one way in which persons act freely, contrasting acting rationally with acting because of uncontrolled passions.

Some, such as Hume, limit the scope of rationality severely, allowing it to characterize mathematical and logical reasoning, but not to underlie normal empirical processes of belief-formation, nor to play an important role in practical reasoning or ethical or aesthetic deliberation. Hume’s notorious statement in the Treatise that ‘reason is the slave of the passions, and can aspire to no other office than to serve and obey them’ is a deliberate reversal of the Platonic picture in which reason (the charioteer) dominates the rather unruly passions (the horses). However, see also dual process theory.