According to Scripture, God designs us in such a way that moral knowledge is natural. In Romans 2:14-16, Paul says the moral law is “written on our hearts,” that is, through the conscience, described by Calvin as “a certain knowledge of the law by nature,”^[2] so that all are without excuse.

Natural law is that apprehension of the conscience which distinguishes sufficiently between just and unjust, and which deprives men of the excuse of ignorance, while it proves them guilty by their own testimony.
—John Calvin, Institutes, Bk. II, Chap. 2, xxii.

In Part 2 of this series, we looked at the content of natural law, which is the Ten Commandments (aka the Decalogue) and the basic design of human nature. One might think that we learn the Ten Commandments by reading the Bible. But if the Bible were the only source of moral knowledge, only a very small percentage of the human race would know right from wrong. And as we will see, the Bible itself doesn’t claim that people know right from wrong only by reading it. But how do we explain how we know the Decalogue? In Part 3, we turn to how we know right from wrong.

To add to the difficulty, doing the right thing often occurs in a bewildering context in which justice doesn’t seem to prevail (Jer. 12:1; Hab. 1:13). Many suffer and die for doing the right things. So we can’t base moral knowledge upon who lives a long and contented life, or a short, difficult one.

Not only do we often fail to see justice prevail, we may find ourselves in circumstances in which being moral is dangerous. Austrian author Stefan Zweig describes a harrowing situation in post-WWI Salzburg during a period of rampant inflation, where in order to survive, one had to be immoral.

A man who respected the food rationing system starved; only one who disregarded it brazenly could eat his fill. A man schooled in bribery got ahead; he who speculated, profited. If a man sold appropriately to the buying price, he was robbed, and if he calculated carefully, he was cheated. Standards and values disappeared during this melting and evaporation of money; there was but one virtue: to be clever, shrewd, unscrupulous, and to mount the racing horse rather than be trampled by it.[1]

The situation Zweig describes challenges those who believe moral knowledge derives from our environment. Saying we obtain moral knowledge from the society or culture we live in has disturbing implications—not just in periods of civilizational collapse as in post-war Austria, but also in contexts in which being immoral is ingrained in the culture we inhabit, such as a street gang, the Mafia, or a corrupt society (Gen. 19).  Moral knowledge must have a more stable basis than what we experience most of the time.

Moral Knowledge

So we can’t simply assume people know, we must at least explain how they know, and especially, why they so often violate what they know, which complicates the question even more.

Yet it is here that natural law by itself is inadequate. We see this most clearly in Aristotle’s wrestling with moral responsibility in the Nichomachean Ethics. He does in fact claim that every human being is responsible for any wrong he commits, unless his action is forced by something outside himself (a blast of wind), or he’s ignorant of the circumstances (1110b25). Yet his explanation of how it is that people can be held responsible is through an indirect argument: If we say that vice isn’t our fault, then neither is virtue, yet this seems preposterous. He concludes:

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