This was originally going to be a post about Kant's 1793 On the common saying: "That may be True in Theory, but it does not Apply in Practice" (often referred to as just "Theory and Practice"), but I ended up with a satisfactory argument for Kant's views within the Groundwork alone.
I'll place the sections of Theory and Practice that I was originally going to discuss in the
afternotes. I don't have an academy-numbered edition, so the page references will be for "Kant's
Political Writings", ed. Hand Reiss, trans. H. B. Nisbet (Cambridge 1970, ISBN
In a blog post earlier this year, I objected to the common claim that, for Kant(ians), the presence of any emotions (or contravening inclinations) compromises the moral worthiness of an action:
[...] in context, Kant is more subtle: he claims only that cold-hearted cases are the clearest cases of moral worth, and leaves it open as to whether agents may act from duty while possessing (non-contravening) inclinations. Thus, the mere presence of emotions like happiness are not necessarily a worth-or-praise diminishing factor for Kantians; they only become so if shown to contravene.
However, I didn't say what I meant by "more subtle." What I should have said is this: that Kant's arguments in the Groundwork holistically support the view that one can act from duty (i.e., in a morally praiseworthy fashion) without needing to be "cold-hearted." Where Kant implied, I'm going to try to be explicit.
Happiness, despite not being the basis for morality, is an end that all rational beings necessarily possess:
[...] there is one end that can be presupposed as actual in all rational beings (in so far as imperatives suit them, namely as dependent beings), and thus one purpose that they not merely can have, but that one can safely presupposed they one and all actually do have according to a natural necessity, and that is the purpose of happiness. (GMS 4:415)
Thus, for Kant, it is simply not the case that rational beings can ultimately act against the end of happiness: we might choose to do so in instances (and, in fact, are frequently commanded to do so by the moral law), but we can never extinguish the end itself.
Only those actions that conform with duty and are done from it are morally praiseworthy:
It is just there that the worth of character commences, which is moral and beyond all comparison the highest, namely that he be beneficent, not from inclination, but from duty. (GMS 4:399)
In other words, we would be incorrect to praise someone's virtue should they either fail to act in conformity with duty (i.e., do the wrong thing) or act in conformity, but do so for the wrong reasons (e.g., that they'll be rewarded).
If rational beings ought to act from duty, then it must be the case that they can, per ought implies can.
Of all of the steps in this argument, this one is the only one not explicitly made in the Groundwork. Instead, it belongs his general thought on agency: that it just isn't sensible to talk about what an agent ought to do if they can't, in fact, do that thing.
Rational beings indeed ought to act from duty, as the "concept of duty contains that of the good will" (GMS 4:397) and the good will is the the only thing that is good without qualification (GMS 4:393).
These are some of Kant's standard talking points in the Groundwork. They're controversial, metaethically, but not within the context of this argument.
Therefore, it must be the case that rational beings can act from duty. (Modus ponens)
Therefore, the fact that all rational beings necessarily have happiness as an end cannot essentially conflict with their capacity to act from duty.
And so we have it explicitly: Kant cannot hold that the mere presence of emotions threatens the moral praiseworthiness of agents, as this would compromise the practical (i.e., action-driving) nature of morality — in order for morality to be the sort of thing that tells us how to act, it must first be the case that we can act according to it.
Here were some of the segments of Theory and Practice that I was originally going to use:
Rational beings cannot renounce their end of happiness:
[...] man is not thereby expected to renounce his natural aim of attaining happiness as soon as the question of following his duty arises; for like any rational being, he simply cannot do so. (KPW 64)
Rational beings are entitled to seek their happiness, provided that it does not come into essential conflict with their duties:
For I must be certain that I am not acting against my duty. Only then am I entitled to look round for happiness, in so far as I can reconcile it with the state that I know to be morally (not physically) good. (KPW 68)
Indeed, "cold-hearted" cases are all but impossible, in the sense that we are always aware of some other consideration of ours:
I willingly concede that no-one can have certain awareness of having fulfilled his duty completely unselfishly. For this is part of inward experience, and such awareness of one's psychological state would involve an absolutely clear conception of all the secondary notions and considerations which, through imagination, habit, and inclination accompany the concept of duty. And this is too much to ask for. (KPW 69)
Being aware of our ability for self-denial and conscientiously combating is sufficient for moral praiseworthiness:
Perhaps no recognized and respected duty has ever been carried out by anyone without some selfishness or interference from other motives [...]. But by careful self-examination, we can perceive a certain amount. We can be aware not such much of any accompanying motives, but rather of our own self-denial with respect to many motives which conflict with the idea of duty. In other words, we can be aware of the maxim of striving towards moral purity. And this is sufficient for us to observe our duty. (KPW 69)