The sum total of the ethical precepts implanted by God in the rational nature of man, through the observance of which he, as a free intelligent being, might attain his natural destiny.
It is that universal, unchangeable, eternal law which Saint Paul says is indelibly written by the Creator in our hearts or in our very nature, urging us to observe the moral order, to do good, and avoid evil.
The ultimate basis and source of the natural law is the eternal law or Divine reason ordering and directing all things in accordance with their natural inclinations to their proper acts and ends.
The expression or participation of this eternal law in free rational creatures is the natural law which may be summed up in the general prescription to do good and avoid evil, to live in conformity with right rational nature, or to observe the moral order as Divinely constituted and sanctioned.
The conduct of all men is subject to the moral law which, as to its primary or general principles at least, is naturally promulgated or known through human reason.
Because of the consequences of original sin, Revelation alone can provide complete knowledge of its secondary and especially of its remote principles as applied to the complexities of life.
The due regulation of our free actions in conformity with its prescriptions secures their right ordering in which consists the natural perfection of our rational nature, and which at the same time constitutes a necessary condition for supernatural perfection, for, according to Saint Thomas, "Just as grace presupposes nature, the Divine Law presupposes the natural law."
New Catholic Dictionary