Rationality, practical
Authors: Hampton Jean 
Published: September 2018

Whereas theoretical reason is that form of reason that is authoritative over belief, practical reason is that form of reason that applies, in some way, to action: by either directing it, motivating it, planning it, evaluating it or predicting it. Accounts of practical reason include theories of how we should determine means to the ends we have; how we should define the ends themselves; how we should act given that we have a multiplicity of ends; how requirements of consistency should govern our actions; and how moral considerations should be incorporated in our deliberations about how to act.
Economics has provided, in recent times, what many regard as the most compelling portrait of practical reason, called ‘expected utility theory’ (hereafter ‘EU theory’). On this theory, rational action is that action which yields the highest expected utility, which is calculated by measuring the utility – or the ‘goodness’ or ‘badness’ – of the possible outcomes of the action, multiplying the utility of each outcome by the probability that it will occur, and, finally, adding together the results for all the possible outcomes of each action. The action that has the highest expected utility is the rational action. Other technical representations of practical reason have been explored in the branch of social science called ‘game theory’, which studies ‘strategic’ situations in which the action that is rational for any agent depends in part on what other agents do.
A theory of practical reason can have one or more of several different goals. If it sets out how human beings actually reason, it functions as a descriptive theory of reasoning. If it sets out a conception of how our reasoning ought to proceed, it functions as a normative theory of reasoning. Theories of reason can also be about actions themselves: if a theory presents a conception of the way our actions should be intelligible or consistent or useful (regardless of the quality of the deliberation that preceded it), it functions as a (normative) theory of behavioural rationale. If it merely presents an account of consistent action that allows us to predict the behaviour of an agent whose previous actions fit this account of consistency, it functions as a descriptive theory.
One might say that whereas theoretical reason is supposed to pursue truth, practical reason is supposed to pursue some sort of good or value in human action. Theories that take rational action to be that which achieves, furthers or maximizes (what is regarded as) good, are consequentialist or teleological theories. Theories that believe rational action must sometimes be understood as action that has an intrinsic value or ‘rightness’ regardless of how much good it will accomplish or manifest, are non-consequentialist or non-teleological conceptions of reason. If the theory defines reason as that which serves ends defined by something other than itself, it is an instrumental conception. If it allows reason to have a non-instrumental role, itself capable of establishing at least some of our ends of action, it is setting out a non-instrumental conception. Theories of practical reason that recognize the existence of a special moral reasoning procedure tend to represent that procedure as non-instrumental.
Philosophers have disagreed about whether practical reason gives us a way of reasoning prior to choice that can actually motivate us to behave in the way that it directs. Many believe it lacks motivational power, so that it can only give us authoritative directives that must be motivated by something else (for example, by our desires). Finally, the study of practical reason also considers the variety of ways in which one can fall short of being rational; and issues about the nature and possibility of irrational ‘weakness of will’ have been central to this discussion.