Christine Korsgaard has argued that Humean views about action and practical rationality jointly imply the impossibility of irrational action. According to the Humean theory of action, agents do what maximizes expected desire-satisfaction. According to the Humean theory of rationality, it is rational for agents to do what maximizes expected desire-satisfaction. Thus Humeans are committed to the impossibility of practical irrationality – an unacceptable consequence.
I respond by developing Humean views to explain how we can act irrationally. Humeans about action should consider the immediate motivational forces produced by an agent's desires. Humeans about rationality should consider the agent's dispositional desire strengths. When (for example) vivid sensory or imaginative experiences of desired things cause some of our desires to produce motivational force disproportional to their dispositional strength, we may act in ways that do not maximize expected desire-satisfaction, thus acting irrationally. I argue that this way of developing Humean views is true to the best reasons for holding them.