Summa Theologiae Moralis, vol. I: De Principiis

Not only must we inquire into the subjective criterion of morality, or the norm by which it is discerned, but also into the objective criterion and the foundation of the intrinsic discrimination between good and evil, or the ontological principle from which the rule of morality has its origin and force, and on which right reason relies for judging what is good or evil.

127. Thesis. The objective foundation of morality, the proximate of the rule of reason, is human nature with its end and essential relations; the ultimate, of the eternal law, is the divine essence itself.

Many recent authors say that human rational nature as such in itself is the proximate norm of morality; — or even human nature completely considered with all its essential relations, namely as rational (and not merely sentient), as social, and as created¹; — some even add that the fundamental norm or the foundation of the eternal law and the will of God itself². But all these confuse the norm with its objective foundation. — On the contrary, some deny that morality depends on the ultimate end to which human nature is nevertheless ordered³, or even on the essential order of things⁴. But we must say:

1° Human rational nature as such, or even completely considered with its relations, is not the formal rule and measure or criterion by which morality is distinguished (and even if one wished to call it a rule, it would at least not be the proximate rule), but the objective foundation according to which reason discerns what is good or bad: for from this the object or act is discerned to be good or bad because it has a relation of fittingness or unfittingness with rational nature. Whence the near norm and subjective criterion of morality is only the practical judgment of reason. And therefore S. Thomas distinguishes between physical good and moral good, between works of nature and works of will, and says that nature is the rule of natural works, reason the rule of acts of will⁵.

For reason is right from its conformity to the eternal law. The eternal law is made known through creatures, and especially through human nature. Therefore human nature stands as a medium between the eternal law and reason, and in this sense it is said to be so, but it is not a rule, except fundamentally, i. e., as the foundation of the rule of reason.

2° Consequently, even the ultimate end of man to which rational nature is itself naturally ordered, and the very essential order of things or the objective order arising from the necessary relations of the human agent to various entities, for the same reason, can be called the objective foundation of morality. They are a rule fundamentally, or the foundation of the rule of reason⁶.

3° Human nature, however, and the essential order of things, are not the fundamental rule nor the foundation of the supreme rule and the eternal law, because before the idea of God and independently of it, created essences and the order among them are nothing at all and have no being either real or ideal; whence the foundation of the eternal law, as of all things, is the divine essence alone, which is the absolutely ultimate end.

128. Remark. — Lest a difficulty arise from the fact that we define good and evil according to conformity or disformity, whether to rational nature, or to reason, or to the ultimate end; for these all come down to the same thing. For good is that which suits some nature; rational nature is constituted by reason; nature is also the principle of operation tending toward its proper end. Therefore it is the same to say: moral good suits rational nature, or reason, or the ultimate end of human nature.