Kant took up what was historically Plato’s quest for a single root principle of ethics, and seems to have succeeded in getting further with it than Plato did. “Categorical imperative” is a less-than-beautiful name for a truly beautiful concept, elegant in its simplicity. Its essential feature is a sort of lifting of concrete choices into universality, in the very strong sense of something said unconditionally of all possible cases. It encourages us to think of a good choice as one that would be good if applied by everyone all the time. This does not positively answer the question what is good or what we should do, but it does help us toward an answer, by ruling out many possibilities.
As with the Platonic idea of the Good, the categorical imperative is conceived as a unique, highly abstract value that should apply to all cases whatsoever. Unlike Aristotle, Plato wanted to hold out for a rigorously single idea of the Good, even though that made it undefinable. Kant also wanted a single ethical principle, but his version is an abstract procedural criterion related to notions of procedural justice, rather than an abstract aim or end like Plato’s. In this case at least, the fact that Kant’s unique principle is procedural makes it possible to say more about it.
Aristotle called Plato’s suggestion of a single principle beautifully said, but went on to note its weaknesses. Plato in effect suggested that contrary to appearances, it is descriptively true that all things aim at the Good. The strong point of this is its inclusiveness. It is most meaningful as a sort of edifying poetic cosmological statement, like when Leibniz said we live in the best of all possible worlds. Cosmically edifying poetry may help us feel at home in the world and thus have a positive effect on our emotions that may aid ethical development, but it does not give us actual ethics. If with Plato we want to consistently say that all things already aim at a unique Good, then that same notion of Good by its very inclusiveness cannot be the kind of thing that could be used as a selective or normative criterion that would actually rule out some possible courses of action, and enable us to tell better from worse. In addition to the issue with its inclusiveness, the explanatory role of Plato’s idea of the Good would be in conflict with its use as a distinguishing criterion of normative selection. (Instead, the strong points of Plato’s ethics lie in epistemic modesty and the ideal of rational dialogue.)
Kant’s attempt to articulate a single root principle of ethics fares better than Plato’s in this regard. As a meta-level strategy, instead of saying a description is true, the categorical imperative says that a procedural criterion should be applied. Kant’s different formulations of the categorical imperative basically represent different informal tests for strong universality. While they are still not sufficient to tell us positively what to do in particular cases, such tests can help us deliberate, because they are sufficient to rule out many alternatives.
Kant developed a unique kind of strong higher-order moral necessity, as a sort of function whose value as a function can be rigorously evaluated, while leaving the evaluation of its arguments (the particulars to which it is applied) as a separate task.
I take it to be a strength of Kant’s approach that the categorical imperative does not actually dictate, but only guides what to do in any particular case. At an extremely abstract level, the categorical imperative has a kind of ironclad moral necessity, as Kant liked to remind us. But this still leaves open the question of its application to particulars, implicitly requiring something like Aristotelian practical judgment to fill the gap.
It does seem strange that Kant so downplayed the work and ambiguities involved in the application of very abstract principles to complex particulars. On the other hand, the categorical imperative was probably the most important new development in ethics since Aristotle. In any case, Kant chose to emphasize a pure procedural principle that can be both determined with necessity, and used to test potential maxims and rule out those that lack plausibility as strong universals, independent of all questions of our interpretation of the particulars to which the principle is to be applied.
Hegel’s formulation of mutual recognition ultimately aims at the same kind of ethical universality as the categorical imperative, while recasting the Kantian transcendental and the metaphysics of morals into something that begins from — but does not remain limited to — concrete social relationships, considered as instances of the universal community of rational beings. While the mature Hegel often criticized Kant’s formalism, the young Hegel had been greatly impressed by Kantian ethics. Hegel’s tendency to superficially polemicize against Kant needs to be balanced against deeper resonances, and the fact that — along with Aristotle — Kant got more pages in Hegel’s History of Philosophy than anyone else.