Good is the first thing that is apprehended by practical reason which is directed
toward action... 'Good is that which all things seek.' Hence, the first precept of
law is that good is to be done and pursued, and evil is to be avoided. All the
other precepts of the law of nature are based on this... all things to which man
has a natural inclination are naturally apprehended by the reason as good and
therefore as objects to be pursued, and their opposites as evils to be avoided..
natural law contains those things that preserve human life and prevent its
destruction... the natural law is said to contain 'what nature has taught all
animals,' such as the union of man and woman, the education of children,
etc.Thirdly, there is in man a natural inclination to the good of the rational
nature..."
St. Thoman Aquinas "The Natural Law" Part 2, Qu. 94.2
The Summa Theologiae 1266-1268