"Purity" in Morals

Here is a claim I think is obviously true, but which I'm not sure I've ever seen anyone state clearly, and which in any case doesn't seem to be as significant to people as I think it should be: Decision theory is the pure doctrine of prudence, in Kant's sense: it is the parallel of what Kant does in the first two parts of the "Groundwork for the Metaphysic of Morals". This claim I mean strictly and rigorously; Kantian ethicists who mock decision theory are as misguided as utilitarians who denigrate the Moral Law itself.

Here is why I think people miss this obvious truth: Decision theory is not "pure", in Kant's sense, because it is not independent of psychology; it is not entirely unempirical, despite being largely mathematized. But Kant's own work is also not independent of psychology, as anyone who has taught Kant's own examples from the second section of the "Groundwork" is well aware: Kant helps himself to just a little bit of psychology, to the claim that (as Plato had already said in the second book of the "Republic") each human has many desires, and no one can satisfy these desires without assistance from others. Kant doesn't realize how much of a cheat this is, and so keeps up the artificial attempt to build ethics on this thin of an account of the human -- it is thus no accident that the full "Metaphysic of Morals" has so much rubbish in it, as Kant was building on sand. Hegel's ethics is free of this artificial limitation that Kant had laid down for ethics, but still shows all the limitations of Hegel's own parochialisms (sexism, racism, nostalgia for guilds and wariness of mass suffrage). So the obvious lack of "purity" in decision theory is not a barrier to granting it status parallel to Kant's own (hypothetical) derivation of the Moral Law in the first two sections of the "Groundwork": that work is itself not really pure, once it has been properly understood. By Kant's own standards, nothing is really "pure": and so what is reasonably called "pure" is not going to meet Kant's strictures.

That there is still something to the "pure"/"empirical" distinction I think is clearly shown by the case of decision theory: decision theory is not like the mass of information that is involved in making actual decisions; avoidance of dutch books in decision theory is clearly akin to crafting reductio proofs, as anyone who has played a little with both must realize. There's some sort of categorical difference between keeping a dutch book and simply making an unwise choice: this is what I think the "pure"/"empirical" distinction should aim to make clear. But even the terms used in labelling the distinction make this difficult. Hence much becomes muddled when thinking about practical philosophy.

Unrelated: I wrote a dissertation, you can find it open-access here.