irrationality unreasonableness. Whatever it entails, irrationality can characterize belief, desire, intention, and action.
Irrationality is often explained in instrumental, or goal-oriented, terms. You are irrational if you (knowingly) fail to do your best, or at least to do what you appropriately think adequate, to achieve your goals. If ultimate goals are rationally assessable, as Aristotelian and Kantian traditions hold, then rationality and irrationality are not purely instrumental. The latter traditions regard certain specific (kinds of) goals, such as human well-being, as essential to rationality. This substantialist approach lost popularity with the rise of modern decision theory, which implies that, in satisfying certain consistency and completeness requirements, one’s preferences toward the possible outcomes of available actions determine what actions are rational and irrational for one by determining the personal utility of their outcomes. Various theorists have faulted modern decision theory on two grounds: human beings typically lack the consistent preferences and reasoning power required by standard decision theory but are not thereby irrational, and rationality requires goods exceeding maximally efficient goal satisfaction.
When relevant goals concern the acquisition of truth and the avoidance of falsehood, epistemic rationality and irrationality are at issue. Otherwise, some species of non-epistemic rationality or irrationality is under consideration. Species of non-epistemic rationality and irrationality correspond to the kind of relevant goal: moral, prudential, political, economic, aesthetic, or some other. A comprehensive account of irrationality will elucidate epistemic and non-epistemic irrationality as well as such sources of irrationality as weakness of will and ungrounded belief.
See also DECISION THEORY, JUSTIFICATION, RATIONALITY. P.K.M.