The Humean theory of practical irrationality.

Author:Sinhababu, Neil

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. As long as one's answer to "What will agents do?" is the same as "What is it rational for agents to do?"--as is the case with the two Humean theories laid out above--the objection will apply.

  2. Korsgaard's Objection

    Korsgaard provides an instance of a general argument against accepting accounts of motivation and rationality as neatly matched as the two components of the double-Humean view. She objects that such accounts make practical irrationality impossible:

    The problem is coming from the fact that Hume identifies a person's end as what he wants most, and the criterion of what the person wants most appears to be what he actually does. The person's ends are taken to be revealed in his conduct. If we don't make a distinction between what a person's end is and what he actually pursues, it "will be impossible to find a case in which he violates the instrumental principle (1997: 230). Korsgaard spells out the "instrumental principle" as follows in a later work: "[P]ractical rationality requires us to take the means to our ends." (5) She adds that "some philosophers regard the ends in question as things desired" (2008: 5). This group would seem to include Hume and contemporary Humeans about reasons. One might understand the Humean theory of rationality as an elaboration of the instrumental principle that delivers precise answers as to which action is rational. (6) Treating the performance of an action as a means to satisfying our desires, an account of rationality based on the instrumental principle would present the action that provides the greatest expected desire satisfaction as the rational one.

    Korsgaard's objection to the double-Humean view is that it makes practical irrationality impossible by giving matching accounts of rationality and motivation. If a theory of motivation says that one will perform a particular action, and a theory of rationality says that it is rational for one to perform that action, the conjunction of the theories will imply that one acts rationally. According to Korsgaard, Hume not only says that "people don't in fact ever violate the instrumental principle. He is actually committed to the view that people cannot violate it" (1997: 228). To avoid this consequence, we might reject at least one component of the double-Humean view.

    The sort of practical irrationality the double-Humean needs to explain is not merely a matter of doing the wrong thing because of false belief, like drinking gasoline because you mistakenly believe that it is gin. (7) In these cases, we might say that there was a reason for you to do otherwise, but that by itself does not make you irrational. As Korsgaard says, "the possibility of mistake is not in general very interesting" (1997: 227). She explains why in an earlier paper: "judgments of irrationality, whether of belief or action, are, strictly speaking, relative to the subject's beliefs. Conclusions drawn from mistaken premises are not irrational." (8) One need not develop the double-Humean view in any especially sophisticated way in order to account for the failure to do what there is most reason for you to do, at least when we are talking about the kind of reasons that are tied to actual (and not expected) desire satisfaction.

    The trouble is to account for the sort of failure to act rationally that can occur even when the agent has no relevant false beliefs. Korsgaard offers an example:

    Howard, who is in his thirties, needs medical treatment: specifically, he must have a course of injections, now, if he is going to live past fifty. But Howard declines to have this treatment, because he has a horror of injections. Let me just stipulate that, were it not for his horror of injections, Howard would have the treatment. It's not that he really secretly wants to die young anyway, or anything fancy like that. Howard's horror of injections is really what is motivating him.... [L]et's again stipulate that he has not miscalculated or made a mistake. He sees that, if he were governed by considerations of prudence, he would have the injections: he agrees that a long and happy life is a greater good than avoiding the injections. But he still declines to have them: he chooses "his own acknowledg'd lesser good" (1997: 227). Howard knows the consequences of his actions, even as he rejects the injections that will save his life. In refusing the injections, Howard acts irrationally. If double-Humeans...

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