IN "THE NORMATIVITY OF INSTRUMENTAL REASON," Christine Korsgaard presents a problem for those who accept similarly structured Humean views of both action and rationality. (1) (I will call the conjunction of views she criticizes the double-Humean view.) Korsgaard contends that the double-Humean view implies the impossibility of irrational action, as it claims that we can only perform the actions that it deems rational. First I will develop Humean views of rationality and action so as to display the force of Korsgaard's objection. Then I will respond by showing how double-Humeans can develop their view to account for just as much practical irrationality as there is.
Double-Humeans can answer Korsgaard's objection if their views of action and rationality measure agents' actual desires differently. What determines what the agent does should be the motivational forces that desires produce in the agent at the moment when she decides to act. That is when her desires play their causal role in determining action. What determines what it is rational to do should be the agent's dispositional desire strengths. Our normative intuitions about rationality concern these states. Since the action that desire motivates us most strongly to do at the moment of action may not be the action that would best satisfy our dispositional desires, irrational action is possible. This way of filling out the double-Humean view is true to the best reasons for accepting both of the theses that compose it, and lets us better understand the nature of irrational action.
1. The Double-Humean View
Humeans about motivation hold that desire is necessary for motivation, while Humeans about reasons hold that our reasons for action are grounded in our desires. These theories have not always been elaborated so as to clearly answer questions about which action someone will perform or which action it is rational to perform. I will elaborate Humean theories about motivation and reasons in a way that answers these questions, as doing so helps to display the force of Korsgaard's objection.
According to the Humean theory of motivation, the only states of mind necessary for motivating action are a desire and a belief that an action will bring about the desired state. (2) This is a nonnormative theory describing the psychological states that cause action. If we add up the motivational forces favoring or disfavoring each action, each force being the product of a desire with a particular strength and a belief about how acting will affect the probability of desire satisfaction, we arrive at the following theory about what agents will do:
Humean theory of action: Agents do whatever maximizes expected desire satisfaction. (3)
The notion of "expected desire satisfaction" here is familiar from decision theory, and can be characterized as a subjective-probability-weighted average of the extent to which various outcomes would satisfy the agent's desires.
According to the Humean theory of practical reason, agents have reason to act insofar as the action promotes the satisfaction of their desires. (4) This is a normative theory in which reasons give actions pro tanto justification. Weighing and adding up the reasons as we previously did with the motivational forces, in terms of desire strengths and changes in probabilities of desire satisfaction, we can produce the following theory about what it is rational for agents to do:
Humean theory of rationality: it is rational for agents to do whatever maximizes expected desire satisfaction.
Henceforth I will take double-Humeans to be committed to the conjunction of the Humean theory of action and the Humean theory of rationality. I beg the indulgence of readers who see better options for Humeans regarding what agents will do and which action is rational, as my purpose in developing Humean views on these two questions is mainly to demonstrate the force of Korsgaard's objection. …