In Mere Christianity, C. S. Lewis insisted that the Moral Law is different from the Law of Nature and possesses a different kind of reality independent of us because of its cognitive and conative nature.
…what we usually call the laws of nature–the way weather works on a tree for example–may not really be laws in the strict sense, but only in a manner of speaking. When you say that falling stones always obey the law of gravitation, is not this much the same as saying that the law only means ‘what stones always do’? You do not really think that when a stone is let go, it suddenly remembers that it is under orders to fall to the ground. You only mean that, in fact, it does fall. In other words, you cannot be sure that there is anything over and above the facts themselves, any law about what ought to happen, as distinct from what does happen. The laws of nature, as applied to stones or trees, may only mean ‘what Nature, in fact, does’. But if you turn to the Law of Human Nature, the Law of Decent Behaviour, it is a different matter. That law certainly does not mean ‘what human beings, in fact, do’; for as I said before, many of them do not obey this law at all, and none of them obey it completely.
….The Moral Law, or Law of Human Nature, is not simply a fact about human behaviour in the same way as the Law of Gravitation is, or may be, simply a fact about how heavy objects behave. On the other hand, it is not a mere fancy, for we cannot get rid of the idea, and most of the things we say and think about men would be reduced to nonsense if we did. And it is not simply a statement about how we should like men to behave for our own convenience; for the behaviour we call bad or unfair is not exactly the same as the behaviour we find inconvenient, and may even be the opposite. Consequently, this Rule of Right and Wrong, or Law of Human Nature, or whatever you call it, must somehow or other be a real thing–a thing that is really there, not made up by ourselves. And yet it is not a fact in the ordinary sense, in the same way as our actual behaviour is a fact. It begins to look as if we shall have to admit that there is more than one kind of reality; that, in this particular case, there is something it is not a fact in the ordinary sense, in the same way as our actual behaviour is a fact. It begins to look as if we shall have to admit that there is more than one kind of reality; that, in this particular case, there is something above and beyond the ordinary facts of men’s behaviour, and yet quite definitely real–a real law, which none of us made, but which we find pressing on us.
Let us sum up what we have reached so far. In the case of stones and trees and things of that sort, what we call the Laws of Nature may not be anything except a way of speaking. When you say that nature is governed by certain laws, this may only mean that nature does, in fact, behave in a certain way. The so-called laws may not be anything real–anything above and beyond the actual facts that we observe. But in the case of Man we saw that this will not do. The Law of Human Nature, or of Right and Wrong, must be something above and beyond the actual facts of human behaviour. In this case, besides the actual facts, you have something else–a real law which we did not invent and which we know we ought to obey.
Lewis’ anti-realism is evident in his opposition of the idea that mathematical objects exist [see quote by Craig]. However, his argument for the existence of the moral law was crucial for his establishment of the moral law argument for the existence of God. The moral law, certainly, cannot depend on humans; it has to be “beyond the actual facts of human behaviour”. Lewis cannot see the moral law as being similar to the laws of nature because of the element of choice that humans have and the fact that they usually do not obey the moral law. It is not a deterministic part of their being; of course, it cannot be, for man is a free being. It is not the same as the involuntary laws of human physiology. They are voluntary laws, by definition. But, why can’t they be intrinsic to volition (persons) in the same way that natural laws are intrinsic to mechanism (things)? Thus, they are not deterministic (as in mechanical laws), but conscientious (involving freedom of choice).
..for when Gentiles, who do not have the law, by nature [Gr.phusis] do the things in the law, these, although not having the law, are a law to themselves, who show the work of the law written in their hearts [intrinsically], their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them). (Rom 2:14-15, parenthetics mine)
The moral law is the way persons are related to each other, but persons are volitional beings; therefore, there is the a priori understanding of the moral law in the sense of an ought. In its fundamental nature, it is love. However, in the complexities of human relations (family, marriage, society, nationality, etc), the implications of the ought are multiplied. Animals are free of these. But, man created in the image and likeness of God is morally accountable. Volitionality cannot be denied, for it explains why people think morally and make moral choices. Sin is sin because it violates the unity of persons and creates disunity and alienation among them; it is a violation of the eternal order of love that is the order of unity among persons whose Head is God; it is violation of persons, and is, ultimately, irreverence towards God; therefore, sin is not a temporal issue but a cosmic one. It is personal. The punishment/wages/fruit of sin is divine abandonment into Godless alienation–eternal death:
…it is a righteous thing with God to repay with tribulation those who trouble you, and to give you who are troubled rest with us when the Lord Jesus is revealed from heaven with His mighty angels, in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ. These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power (2Th 1:6-9, emphatics mine)
If the law was just extrinsic to all relations then “Can a man be profitable to God, though he who is wise may be profitable to himself? Is it any pleasure to the Almighty that you are righteous? Or is it gain to Him that you make your ways blameless?” (Job 22:2-3); and again, “If I have sinned, what have I done to you, O watcher of men? Why have you made me your target? Have I become a burden to you?” (Job 7:20 NIV) But, Justice is the relational act of unity among persons. Where there is alienation, the solution is reconciliation. Where there is no reconciliation (voluntary, not mechanical), there is separation. Therefore, voluntary faith in Christ is essential to reconciliation in which God Himself has taken the first voluntary steps towards reconciliation. The Sacrifice of Christ is the price of this reconciliation which God paid in order to be the suffering member of this covenant. The cost of discipleship is what we pay in order to be part of this reconciliation. Therefore, that He died for all cannot mechanically save all. The bridge is not mechanical; it is volitional. Therefore, it is not just mental faith that saves, faith has to be active, the step towards and in reconciliation. Christ carried His cross, but we can only be His disciples if we carry our cross and follow Him.
Aristotle, Rhetoric, Book I.13
It will now be well to make a complete classification of just and unjust actions. We may begin by observing that they have been defined relatively to two kinds of law, and also relatively to two classes of persons. By the two kinds of law I mean particular law and universal law. Particular law is that which each community lays down and applies to its own members: this is partly written and partly unwritten. Universal law is the law of Nature. For there really is, as every one to some extent divines, a natural justice and injustice that is binding on all men, even on those who have no association or covenant with each other. It is this that Sophocles’ Antigone clearly means when she says that the burial of Polyneices was a just act in spite of the prohibition: she means that it was just by nature.
Not of to-day or yesterday it is,
But lives eternal: none can date its birth.
And so Empedocles, when he bids us kill no living creature, says that doing this is not just for some people while unjust for others,
Nay, but, an all-embracing law, through the realms of the sky
Unbroken it stretcheth, and over the earth’s immensity.