Natural law is a promulgation of the eternal moral law within all of Creation, especially human persons. “The natural law is a participation in us of the eternal law.” (Aquinas, Summa Theologica, I-II, 96, 2.)
The eternal moral law is the Justice inherent in the very Nature of God. Since God is infinitely and perfectly Just, all that He created must be a reflection of that goodness and justice. As the eternal moral law is inherent to God’s Good Nature, so also is the natural law inherent to God’s good Creation. But the highest goods in Creation are created persons, who resemble the Nature of God more so than the rest of creation due to intellect and free will, due to the ability to knowingly choose good over evil, moral over immoral, love over selfishness.
Human persons reflect the natural law in three ways: (1) in our very nature, which is made in the image of God; (2) in our relationship to other human persons, which is an image of the relationship between the three Persons of the Trinity; (3) in our relationship with God. For the existence of God, and the requirement to seek and worship Him, can be known by reason alone, and therefore is part of the natural law.
Natural law is the promulgation of the eternal moral law in all Creation, especially in created persons, in the nature of each created thing, in the ordered relationship between created things, and in the relationship between Creation and God. Moral goodness is inherent to, and understandable from, all Creation. But the nature of created persons is more like the Nature of God than any other created thing. And so the natural law is most clearly perceived within the nature of created persons, within the proper relationships between created persons, and in the relationship between created person and Creator.
Now the natural law is no different, in its requirement to avoid all sin, than the eternal moral law, except that the natural law is the means by which we know the eternal moral law. Therefore, the natural law is the promulgation of the eternal moral law, so that this eternal law of God may be known naturally by created persons, through the use of intellect and free will. The natural law is the promulgation of the eternal moral law by God within the nature and order of Creation. Therefore, the natural law is also universal and immutable, like the eternal moral law.
There is a common false claim about natural law, which says that, because various acts are found in nature among the lower animals, that such acts are ‘natural’ and therefore moral. But that is not the meaning of natural law. The human person is unique among animals, in that he has an understanding, through the gift of a rational soul, of moral truth and of his good Creator. Natural law is based primarily on what reason understands as truly good, given that the human person has reason and free will, and is created by a good God. It is not based on human persons acting like the lower animals.
The behavior of some animals includes acts that would be immoral for human persons, because human persons have the higher gifts of reason and of free will; animals do not have these gifts. Their behavior includes such acts because all of Creation is in a fallen state, and not because God in any way approves of such acts. When did Creation fall? Most probably it fell at the time when some of the angels fell from grace. For a large number of angels were tasked by God with caring for Creation, including the care of the lower animals. When some angels fell, God permitted all of material Creation to fall as well, as a fitting consequence for their grave sins.
The behavior of some animals includes killing their own young, harming and killing members of their own species in disputes over food or over mates, and even at times attacking and killing human persons. Yet no one claims that natural law therefore approves of these same acts. If anyone tried to claim that human persons should behave, in all areas of life, like animals, such a claim would be rejected by all reasonable persons. Therefore, no immoral act can be justified by a consideration of the behavior of animals in nature.
Every act that is immoral under the eternal moral law is also contrary to natural law because natural law is an expression of the eternal moral law and therefore of the will of God for persons made in His image.