The idea of natural law flows from the conviction that reality has weight. Ideas of right, wrong, justice and law, should be based not on opinion, which can change from one time to another, and from place to place, but from what all human beings hold in common, regardless of place, time, race, nationality or religion. That common denominator is human nature, and reason's ability to discover what causes human nature to be fulfilled, prosper and attain to authentic happiness.
The Church's greatest philosopher and theologian, St. Thomas Aquinas (1224– 1274), explains the origins of natural law in the following manner:
Do good and avoid evil
Summa Theologiae I-II question 94, article 2 … Now, as "being" is the first thing that falls under the apprehension simply, so "good" is the first thing that falls under the apprehension of the practical reason, which is directed to action: since every agent acts for an end under the aspect of good. Consequently the first principle of practical reason is one founded on the notion of good, namely, that "good is that which all things seek after."
Hence this is the first precept of law, that "good is to be done and pursued, and evil is to be avoided." All other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man's good (or evil) belongs to the precepts of the natural law as something to be done or avoided.
Since, however, good has the nature of an end, and evil, the nature of a contrary, hence it is that all those things to which man has a natural inclination, are naturally apprehended by reason as being good, and consequently as objects of pursuit, and their contraries as evil, and objects of avoidance. Wherefore according to the order of natural inclinations, is the order of the precepts of the natural law.
Because in man there is first of all an inclination to good in accordance with the nature which he has in common with all substances: inasmuch as every substance seeks the preservation of its own being, according to its nature: and by reason of this inclination, whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law.
Secondly, there is in man an inclination to things that pertain to him more specially, according to that nature which he has in common with other animals: and in virtue of this inclination, those things are said to belong to the natural law, "which nature has taught to all animals", such as sexual intercourse, education of offspring and so forth.
Thirdly, there is in man an inclination to good, according to the nature of his reason, which nature is proper to him: thus man has a natural inclination to know the truth about God, and to live in society: and in this respect, whatever pertains to this inclination belongs to the natural law; for instance, to shun ignorance, to avoid offending those among whom one has to live, and other such things regarding the above inclination.
 EWTN: In this distinction, the practical reason is where we considered truth in reference to its application, whether in the conscience, or in art, music or technology. This is distinguished from speculative reason, where truth is known simply, or in itself (like theology, philosophy, mathematics or physics).
This is, as St. Thomas notes, the first principle of the natural law. Every other principle can, in some way, be derived from it, or reduced to it.
He also notes how “good” is not simply a matter of taste, such as “I love Mozart, and hate Beethoven, whereas, you love Beethoven and hate Mozart,” but is the “good” or “end” of the nature we all hold in common. This is the origin of the idea of the “common good.”
Thus, whereas, individual inclinations vary from person to person, culture to culture, all human beings are inclined to certain things: “to be,” to perpetuate one’s existence through nourishment, the human race through sexual reproduction, to know the truths upon which an authentic human existence depends, including about God, and to live in society with others like himself.
On the principles of the natural law, therefore, depend the common good of every society. Not following them is the ruin not only of individuals but of entire peoples, as history shows.