I think the introduction of rational ethics by Plato and Aristotle was the greatest single event in the history of talking animals on our planet, marking the threshold of a kind of historical cultural adulthood. Before that, there were traditional values; codifications of traditional values into law; and attempts by some people to impose their will on others; but there was no ethics as free and open inquiry into what is right.
Two millenia later, Kant took the next big step, and explicitly argued for the primacy of practical reason. This means that the kind of reasoning involved in rational ethics comes first in the order of explanation, before so-called theoretical reason.
Recently, Brandom’s highly original account of responsibility has closed any remaining gaps, making it possible to explain anything at all in terms that put ethical reasoning first. (See also Expansive Agency; Brandomian Forgiveness.) This also further refines Kant’s concept of the autonomy of reason, allowing for a stronger interpretation that eliminates the last vestiges of a dependency of ethical reasoning on anything external to it. It allows the primacy of practical reason to be fused with the autonomy of reason, resulting in a new kind of completeness of ethical reason.
Of course, any talk about a completeness of ethical reason presupposes a very broad construal of what ethical reasoning is (see also Reasonableness; What and Why; Context). It also requires that we be very careful to avoid taking its completeness in the wrong way. It presupposes a kind of epistemic modesty as a feature of rational inquiry.
Rational ethics stands in contrast to tradition, but as Hegel might remind us, much of the content of tradition turns out to be broadly rational after all, if we disregard its epistemic shortcuts.
The true antithesis of rational ethics is the subordination of values to a supposedly sovereign will — be it the will of God presumed as known; the expressed will of some individual; or a will attributed to an institution like the state, or to a social group. Such appeals to arbitrary will end the possibility of inquiry and dialogue. (See also Euthyphro; Authority, Reason.)