by Mortimer J. Adler, Ph.D.
For a truth to be self-evident it must be beyond the shadow of a doubt. It must be undeniable simply because its opposite is impossible for us to think.
If right desire is desiring what we ought to desire, and if we ought to desire only that which is really good for us and nothing else, then we have found the one controlling self evident principle of all ethical reasoning — the one indispensable categorical imperative. That self-evident principle can be stated as follows: we ought to desire everything that is really good for us.
One may ask why this is self-evident; the answer is that something is self-evident if its opposite is unthinkable. It is unthinkable that we ought to desire anything that is really bad for us; and it is equally unthinkable that we ought not to desire everything that is really good for us. The meanings of the crucial words “ought” and “really good” co-implicate each other, as do the words “part” and “whole” when we say that the whole is greater than any of its parts is a self-evident truth.
Given this self-evident prescriptive principle, and given the facts of human nature that tell us what we naturally need, we can reason our way to a whole series of prescriptive truths, all categorical. Kant was wrong in thinking that practical reason itself can formulate a meaningful categorical imperative, without any consideration of the facts of human nature. It is human nature, not human reason, that provides us with the foundations of a sound ethics.
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