First of all, there must be law; that is, a fixed rule of teaching what is to be done and what is to be left undone. This rule cannot affect the lower animals in any true sense, since they act of necessity, following their natural instinct, and cannot of themselves act in any other way. On the other hand, as was said above, he who is free can either act or not act, can do this or do that, as he pleases, because his judgment precedes his choice. And his judgment not only decides what is right or wrong of its own nature, but also what is practically good and therefore to be chosen, and what is practically evil and therefore to be avoided. In other words, the reason prescribes to the will what it should seek after or shun, in order to the eventual attainment of man’s last end, for the sake of which all his actions ought to be performed. This ordination of reason is called law. In man’s free will, therefore, or in the moral necessity of our voluntary acts being in accordance with reason, lies the very root of the necessity of law. Nothing more foolish can be uttered or conceived than the notion that, because man is free by nature, he is therefore exempt from law. Were this the case, it would follow that to become free we must be deprived of reason; whereas the truth is that we are bound to submit to law precisely because we are free by our very nature. For, law is the guide of man’s actions; it turns him toward good by its rewards, and deters him from evil by its punishments.
Foremost in this office comes the natural law, which is written and engraved in the mind of every man; and this is nothing but our reason, commanding us to do right and forbidding sin. Nevertheless, all prescriptions of human reason can have force of law only inasmuch as they are the voice and the interpreters of some higher power on which our reason and liberty necessarily depend. For, since the force of law consists in the imposing of obligations and the granting of rights, authority is the one and only foundation of all law – the power, that is, of fixing duties and defining rights, as also of assigning the necessary sanctions of reward and chastisement to each and all of its commands. But all this, clearly, cannot be found in man, if, as his own supreme legislator, he is to be the rule of his own actions. It follows, therefore, that the law of nature is the same thing as the eternal law, implanted in rational creatures, and inclining them to their right action and end; and can be nothing else but the eternal reason of God, the Creator and Ruler of all the world. To this rule of action and restraint of evil God has vouchsafed to give special and most suitable aids for strengthening and ordering the human will. The first and most excellent of these is the power of His divine grace, whereby the mind can be enlightened and the will wholesomely invigorated and moved to the constant pursuit of moral good, so that the use of our inborn liberty becomes at once less difficult and less dangerous. Not that the divine assistance hinders in any way the free movement of our will; just the contrary, for grace works inwardly in man and in harmony with his natural inclinations, since it flows from the very Creator of his mind and will, by whom all things are moved in conformity with their nature. As the Angelic Doctor points out, it is because divine grace comes from the Author of nature that it is so admirably adapted to be the safeguard of all natures, and to maintain the character, efficiency, and operations of each. – from Libertas (1888)