The law which they call the eternal law, when considered as it exists in the mind of God, has many different names when considered according to the different things it is applied to…when applied to the law which binds reasonable creatures in such a way that they can plainly perceive it, we call it the law of reason…

Though we sometimes define the law of nature as the way that God has decided each created thing should act, we need to make a careful distinction. We most properly call natural agents those things which obey their laws necessarily, such as the heavens and the elements of the world, which have no choice in what they do, while we call rational beings with a free will voluntary agents, to set apart the two categories. In the same way, it will be helpful if we distinguish the law observed by the one from the law observed by the other—hence my category, the law of reason.

Everyone recognizes the way that natural agents consistently keep one course, statute, and law; yet man has never achieved, nor perhaps ever will, full understanding of their ways.[3. Ibid, ch. 3.]…

Let us return to our earlier plan of exploring the natural way by which we discover the rules of goodness that guide man’s will in all his actions. Just as everything naturally and necessarily desires the highest perfection it is capable of achieving, so does man. Since our happiness is the object of all our desires, we cannot help but wish it. Whatever falls within the scope of human action, the will inclines to it to the extent that our reason judges it better for us, and ultimately better for our happiness. If reason errs, we fall into evil and are deprived of the general perfection we seek. Since knowledge of good and evil is so necessary for right action, all that is left is to ask how we might possess it…

It is agreed that things acting according to their nature all keep to the same course. The general and perpetual voice of mankind is as the judgment of God Himself, since what all men at all times have come to believe must have been taught to them by Nature, and since God is Nature’s author, her voice is merely His instrument. There are any number of duties we must perform that are made clear enough by this rule alone, without any further warrant being needed. The Apostle Paul says that the pagans are a “law to themselves” (Rom. 2:14), meaning that God illuminates all men with the light of reason so that they can know truth from falsehood and good from evil. By reasoning together they learn what the will of God is, without any supernatural revelation, and thus, when they seem to be making their own laws, they are in fact merely discovering His.

Therefore, we may define a law in general as a rule that directs something how to act well…The rule for voluntary agents on earth is the judgment of reason about what things are the best to be done…

In every subject, there are some basic propositions that, once they have been mentioned, we cannot help seeing that they are undeniably true, even without proof. An example of such an axiom is “the greater good should be preferred to the lesser good.” Our natural tendency is to avoid the painful and seek the pleasant. If we ask why we should ignore this tendency, and instead despise the pleasures of sin and rejoice in the struggles of virtue, we never would unless wisdom clearly told us that great goods are worth small difficulties, whereas fleeting pleasures are not worth the unspeakable harms that follow them.[4. Ibid, ch. 8.]