Both Hume and Kant advocated extreme and implausible views of motivation; the same is also true of many of their contemporary followers. The truth about motivation lies in between these two extremes.
A. Hume. According to Hume (Treatise III.i.1.12):
Reason, in a strict and philosophical sense, can have an influence on our conduct only after two ways: Either when it excites a passion by informing us of the existence of something which is a proper object of it; or when it discovers the connexion of causes and effects, so as to afford us means of exerting any passion.
Hume’s examples of these two motivational processes are as follows (Treatise II.iii.3.7):
- The “supposition” that a fruit is “of an excellent relish” may lead one to “desire” or have a “longing” for the fruit.
- The “supposition” that “certain actions” are “means of obtaining” a “desired end” may lead one to “will the performance” of those actions.
Different interpretations of these two processes are possible, but the following interpretation seems to me most plausible:
- In the first process, a belief to the effect that one’s situation has a certain feature triggers a certain disposition to have a desire—specifically, a disposition to desire something that is related in a certain systematic way to that feature of one’s situation.
- In the second process, having both (a) a belief about how a certain available action will promote a certain end and (b) a desire for that end motivates one to perform the action in question.
Thus, the first process presupposes a disposition to have desires, and the second process presupposes a desire for an end. In both cases, Hume insists that there can be nothing “contrary to reason” about having or lacking any such disposition to have desires or any such desire for an end. “It is not contrary to reason to prefer … my own acknowledg’d lesser good to my greater…” (Treatise II.iii.3.6).
Hume’s theory is extreme and implausible because he does not see that some of these dispositions are in a sense rationally required. In particular, consider the disposition of being disposed to desire to perform an act whenever one believes that one ought to perform the act. Arguably, this disposition is necessarily present in any agent who is capable of having beliefs about what she ought to do, and in a non-accidental way avoids acting akratically.
To be a rational agent, one must not just avoid acting akratically by a lucky chance: one must avoid acting akratically in a non-accidental way. So, arguably, this disposition is necessarily present in every rational agent who is capable of having beliefs about what she ought to do. In that sense, lacking this disposition would indeed be “contrary to reason”.
B. Kant. According to Kant, when our motivation is moral (i.e. when we “act from duty”, which as close as we imperfect beings can come to having a “good will”), we are motivated directly by the moral law itself (as explained at the conclusion of Section 1 of the Groundwork).
The moral law, for Kant, is simply a principle of practical reason. That is, it is a way in which the will of any being who is completely rational will be motivated. Such a completely rational being will reject any non-universalizable maxim that comes before their mind, simply because it is non-universalizable; and being motivated in this way, we are told, will inevitably involve one’s adopting certain ends, such as one’s own perfection and the happiness of others (Groundwork, Section 2).
This principle is meant to be a fundamental feature of all completely rational beings, in a way that cannot be explained by the feelings or sentiments that these beings happen to have. Admittedly, when your motivation is moral in this way, certain “moral feelings” will be produced in you – such as “respect” for the law and the like (as explained in the 2nd Critique, Chapter 2). But these feelings are explanatorily downstream of what is fundamental – the primary fact that you are motivated by the moral law itself.
Kant’s theory is extreme and implausible because this idea of being motivated to adopt an end in this “pure” way, in a way that fundamentally owes nothing whatsoever to the feelings and sentiments that the agent has, seems to be a myth. Without feelings and sentiments to work on, practical reason would be empty and lacking in any direction. To use one of Kant’s own images (1st Critique, B 8), it would be like a dove that tried to fly by flapping its wings in a vacuum.
C. An intermediate view. The intermediate view that I have in mind differs from the Humeans’ view because it allows us to be motivated to act directly by our normative beliefs. However, contrary to what Kant maintains, the intermediate view implies that those normative beliefs must themselves necessarily be sensitive, in some way or other, to our feelings and sentiments.
Nonetheless, contrary to what Hume and his followers maintain, this sensitivity of our normative beliefs to our feelings and sentiments can take a holistic form. The normative beliefs of a rational agent will collectively form a body of beliefs that optimally coheres with the totality of the agent’s feelings and sentiments and other beliefs. But there is no requirement that each individual belief must be directly supported by a particular feeling or sentiment that corresponds to that belief. So there is no reason to think that whenever we are motivated by a normative belief, this belief is simply channelling the motivational force of some pre-existing passion.
It doesn’t matter whether this intermediate view is classified as “Humean” or “anti-Humean”. That is a purely terminological question. What matters is that both Kant and Hume and many of their followers had extreme and implausible views about motivation, and that the truth lies in between these extremes.