The contents of the natural law
Radically, the natural law consists of one supreme and universal principle, from which are derived all our natural moral obligations or duties. We cannot discuss here the many erroneous opinions regarding the fundamental rule of life.
Some of them are utterly false—for instance, that of Bentham, who made the pursuit of utility or temporal pleasure the foundation of the moral code, and that of Fichte, who taught that the supreme obligation is to love self above everything and all others on account of self. Others present the true idea in an imperfect or one-sided fashion. Epicurus, for example, held the supreme principle to be, “Follow nature”; the Stoics inculcated living according to reason. But these philosophers interpreted their principles in a manner less in conformity with our doctrine than the tenor of their words suggests. Catholic moralists, though agreeing upon the underlying conception of the Natural Law, have differed more or less in their expression of its fundamental formula. Among many others we find the following: “Love God as the end and everything on account of Him”;
“Live conformably to human nature considered in all its essential respects”; “Observe the rational order established and sanctioned by God”; “Manifest in your life the image of God impressed on your rational nature.” The exposition of St. Thomas is at once the most simple and philosophic. Starting from the premise that good is what primarily falls under the apprehension of the practical reason—that is of reason acting as the dictator of conduct—and that, consequently, the supreme principle of moral action must have the good as its central idea, he holds that the supreme principle, from which all the other principles and precepts are derived, is that good is to be done, and evil avoided (I-II, Q, xciv, a. 2).
newadvent.org/cathen/09076a.htm