Moral law is natural, says . Basil. No one needs to teach it to us; we already know what’s right and wrong. When we violate the basic principles of morality, we not only go against God, we go against the nature he created in us.
We shall not be able to say in self-justification that we have learned useful knowledge in books, since the untaught law of nature makes us choose that which is advantageous to us.
Do you know what good you ought to do your neighbor? The good that you expect from him yourself. Do you know what is evil? That which you would not wish another to do to you. Neither botanical researches nor the experience of plants have made animals discover those which are useful to them; but each knows naturally what is healthy and marvelously takes for its own what suits its nature.
Virtues exist in us also by nature, and the soul has affinity with them not by education, but by nature herself. We do not need lessons to hate illness, but by ourselves we reject what afflicts us. The soul has no need of a teacher to teach us to avoid vice. Thus temperance is praised everywhere, justice is honored, courage admired, and prudence the object of all aims—virtues that concern the soul more than health concerns the body.
Children, love your parents, and you, parents, “do not provoke your children to anger” (Ephesians 6:4). Does not nature say the same? Paul teaches us nothing new; he only tightens the links of nature. If the lioness loves her cubs, if the she-wolf fights to defend her little ones, what shall we humans say when we are unfaithful to the precept and violate nature herself; or the son who insults the old age of his father; or the father whose second marriage has made him forget his first children?