What is law?
A law is any obligation that is placed on a human person and which regulates his choices.
What is natural law?
Natural law is distinguished from Biblical law in the same way that general revelation is distinguished from and special revelation.
What is general revelation?
General revelation is God's revelation of Himself in nature. From the truths of general revelation, we are able to do natural theology.
Does natural law also come from our study of general revelation?
That is correct.
Natural law then, are those laws which we can discover apart from the Bible?
That is correct.
Does the Bible recognize the existence of such a law?
It does. The clearest passage is Paul's teaching in Romans 2:
For when Gentiles who do not have the Law do by nature the things of the Law, these, not having the Law, are a law to themselves, in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them, on the day when, according to my gospel, God will judge the secrets of men through Christ Jesus. (Rom 2:14-16)
What is meant in these verses when it says that the Gentiles do not have the law?
This means that the Gentiles do not have a copy of God's revealed and written law as the Jewish people did. These same people, however, do have the law.
What does Paul mean by "written in their hearts"?
This means that the law which the gentiles have is not a written law as we understand the word "written". Rather, the law which they possess is something in their hearts and minds. They arrive at an understanding of this law by their own reason and conscience which testifies to certain actions as being wrong or right.
Why does Paul say that the Gentiles obey some laws "by nature?"
There is a contrast here between the Gentile who sins "without the Law" and the Jews who sin "under the law". (Rom 2:12) This simply means that the Jews received God's law by way of direct and written revelation from God Himself. The Gentiles, however, never received any special or written revelation from God. Whenever they happen to obey the commands of God, they are doing it by nature; i.e. simply by way of their own reason and conscience. We would use a similar expression if we said that someone knows math naturally; i.e. they never needed a teacher; they just learned it on their own. The word translated "by nature" is φύσει.
ὅταν γὰρ ἔθνη τὰ μὴ νόμον ἔχοντα φύσει τὰ τοῦ νόμου ποιῶσιν οὗτοι νόμον μὴ ἔχοντες ἑαυτοῖς εἰσιν νόμος (Rom 2:14).
Stuart writes that this word φύσει "means the nature or natural state of a thing, the natural condition of any thing; just in the same way as we use the word nature in our own language." He points out that the Greeks used the expression:
ὁ κατὰ φύσιν θάνατος which translated means a natural death or
ὁ κατὰ φύσιν πατήρ meaning our natural father or
φύσιν ἔχει γένεσθαι meaning it naturally happens.
"In the verse before us, Φύσει is equivalent to μὴ νόμον ἔχοντα; i.e., it designates those who were acquainted with the precepts of natural religion only and were destitute of special revelation." source
Wouldn't it be more accurate to say that natural law contains the obligations we are able to discover from general revelation rather than that we learned it on our own?
Yes, this is certainly correct. Strictly on his own, natural man would know nothing about God whatsoever. God's revelation in nature and in the soul of man is what enables people to arrive at natural theology and natural law.
What is an example of a law which we can understand simply by using our own reason?
Immanuel Kant suggested the following. "Act as if the maxim of thy action were to become by the will a Universal Law of Nature." source
What does this mean?
In a sense, Kant is saying the right thing to do is to ask yourself, what if everyone did this? He provides several examples.
A man who is in despair considers whether it would be right for him to take his own life. Obviously, this cannot be the right thing to do since it would mean the end of the human race if taking your own life was the right thing to do.
Consider a man in debt. He must secure a loan, but he knows that he will never be able to repay the loan. Nevertheless, he must have a loan, or he is ruined. To secure the loan, he must vigorously insist that he will repay the loan, all the while knowing in his own mind that he will not. Can this be a right course of action? That this course of action may be conducive to the man's future well-being is not the question. Is it the morally right thing to do? It cannot be, says Kant, because if it were universally regarded as the right thing to do, it would end commerce as we know it.
Then Kant asks us to consider a man who finds within himself some aptitude or skill. In order to use this skill to the advantage of others, he will need to undergo education and training in order to bring it to the level where he can use it for some good. The circumstances of this man, however, are such that he really has no need to subject himself to the rigors of education and training. He has a comfortable living, plenty of money, and really sees no need to get out of his easy chair. Now what if this man's choice to live in ease and never work to better himself became universally the right thing to do? Clearly, society would sink into misery and savagery.
Finally, Kant points to a wealthy man who is surrounded by people in distress and need. Would it be the right thing for this man to take from his wealth and use it to relieve the misery of these people? Granted, it might be something which makes him feel good and produces a great deal of happiness for himself and others but that is not the point here. The question remains, is it the morally right thing to do? To answer this question, we must ask whether it could be made a universal law to which all men and women of all times and places could be subject? Again, Kant points out that this could not be since the man himself may find himself in need some day and if the right thing to do was to ignore the distress of others, then people would be obligated to ignore his distress. source
Is this a useful way of judging our choices?
It is useful as a way of justifying the morality of a given action to those who do not accept Scripture as infallible. Of course, reasoning to moral principles this way is never as sure as using God's written revelation to guide us. Only in the Bible do we find explicit references to what is right and wrong.
Is Kant's categorical imperative Biblical?
It is. Several times, God commands His people to love others in the same way that they love themselves.
'You shall not take vengeance, nor bear any grudge against the sons of your people, but you shall love your neighbor as yourself; I am the LORD. (Leviticus 19:18)
'The stranger who resides with you shall be to you as the native among you, and you shall love him as yourself, for you were aliens in the land of Egypt; I am the LORD your God. (Leviticus 19:34)
Jesus also taught His followers: "In everything, therefore, treat people the same way you want them to treat you, for this is the Law and the Prophets." (Matthew 7:12)