Natural law as an external principle of action
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Like human law, the natural law is an external principle of action. Originally, it exists in the one who promulgates, which seems to be God. Aquinas says, "The promulgation of the law of nature arises because God has placed in the human mind the ability to know it naturally.1 With God as the author of the natural law, the external character of the law is maintained.
At the same time, the natural law is internal to the person, since it is a work of reason.’^ How can it be both external and internal? In the same manner in which a command is both external and internal. The lieutenant commands the private to clean up the mess hall. In relation to the private, the command is something external, since it arises from the lieutenant. On the other hand, within the private himself the command becomes a kind of judgment. As such, it is internal to him.
The same kind of duality applies to the natural law. It originates as a precept of God, that is, an order of reason impelled by an act of will. The immediate effect of God’s will is certain inclinations in nature. As human beings, however, we become aware of these inclinations and thereby become aware of the will of God. This awareness, in us, is a work of reason; it is a judgment about what ought to be done. In God, the precepts or commands are the same as the eternal law. In us, the awareness law; this participation is also the natural law.
A precept, then, is the content of a command. In the one who commands, it is fully practical; in the one who is commanded, it is a judgment at the level of virtual practical knowledge. Even within this judgment, however, the content must be perceived as arising from outside. If the private thinks to himself "I ought to clean the mess hall", but he does not perceive this ought-statement as arising from a superior, then it is not perceived as a command or precept but merely as an ought statement. Even when a person commands himself, he has one part directing another part.
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Similarly, law is an external principle of action; less properly, it is the individual’s awareness of this principle. The individual must be aware that a precept comes from outside; otherwise, he does not perceive it as a precept; he does not perceive it as law. He may know a set of judgments about what ought to be done, but these judgments will have the character of law only insofar as they are perceived as arising from some external command. We might call these judgments moral principles, but we should not call them law.2
The natural law fits this pattern insofar as it is a participation in the eternal law. The natural law and the eternal law are distinct in scope; the natural law is only that part of the eternal law that applies to voluntary human actions. Within this narrower scope, the two are identical in content, for instance, both the eternal law and the natural law prescribe marriage. Precisely as a law, the content of the prescription must be perceived as arising from God; the natural law is our perception of God’s commands as realized in nature. If we know that we should keep sexual relations within marriage, but we do not know that this rule arises from some authority, then we do not perceive it as a law.
Most properly, then, the natural law is a participation in the eternal law. It is the antecedent will of God for human beings, insofar as we have become aware of it. As an act of knowledge – as opposed to the content of this act- the natural law resides in us; as such, it is a work of reason. The content of this work of reason concerns the order of human actions; the origin of this content is found in God. Of course, someone might perceive the content without perceiving that it arises from God. Then this content would still be a law, since it does in fact arise from God, but it would not be perceived as a law; it would be perceived only as what is necessary for the agent directed to some end.
What we have called the classical picture, then, is reaffirmed. God’s will brings about the effect, in creatures, of certain natural inclinations toward certain ends. The good of these creatures is determined by these ends. Human beings are distinctive in that they become aware of these inclinations and ends, thereby becoming aware of their own good. They also become aware of what is necessary to attain these ends, which awareness can be formulated by way of ought-statements. Only when human beings come to perceive these ought-statements as expressing the will of God do they perceive them as law.
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1 II-II, 90,4, ad I. "Promuigatio legis naturae est ex hoc ipso quod Deus earn mentibus hominum inseruit naturaliter cognoscendam."
2 20. See Dewan, "Natural Lights," 298. 303-4; see also Brock, "Legal Character," 82-84. Fortin goes so far as to say that the legal character (r of the natural law is perceived only with revelation, through which we become awAre of the eternal law; see Fortin, "New Rights Theory," 607-11