What is this Moral Law?
It is not our herd instinct. Instinct is nurturing love, sexual instinct, or finding a food source ina strange place. It's a strong desire to act in a certain way. Sometimes helping someone is instinctual. But that is different from feeling you ought to help whether you want to or not. When you hear someone being attacked your instinct of self preservation says run, your herd instinct says "help," and something else says the instinct to help is RIGHT.
The think that judges between the two impulses cannot be either impulse. The Moral Law is the tune; our instincts are the keys. When two instincts collide, the stronger wins, right? Yet the Moral Law sides with the Weaker, usually. you want to be safe more than help the victim. Moral Law still says help. Moral Law calls us to strengthen herd instinct.
Finally, none of our instincts or impulses is always on the side of Moral Law. Our impulses -- nurturing, patriotism, sex, fighting -- cannot be called bad or good. Sometimes we need them. Sometimes they're harmful. Are any notes on a piano right or wrong? But each note can be wrong at the wrong time. Moral Law is the tune dictating when a note is correct and when it is wrong.
One cannot set up an instinct as that which must be followed at all costs without becomming a devil of sorts. Instincts cannot become absolutes. Love for humanity when unchecked by justice can become a devil.
Is Moral Law something that society teaches and nothing more? We're taught the multiplication tables. Does that make them untrue if we're not taught them? Or could we alter the multiplication tables if we liked, to fit society better? No. We learn the Moral Law from adults, society, books, and all sorts of places. But is Moral Law like which side of the road we learn to drive on or is it like mathematics?
First, societies do not differ in basic morality. Over time and distance, the Moral Law remains the same. Second, do you ever think one society's morality is better than another's? Do they make improvements? If not, then there's no moral progress. Progress means changing for the better. Do you think Christian Morality is better than Nazi Morality? Of course we think some moralities are better than others. We look toward reformers who understand morality better than societal leaders. We measure the two societies against some sort of Real Morality. This standard is different than either socieity we're measuring. This Real Morality is something idependent of what people think. Some people can be closer to this idea than others. If your moral ideas can be truer than than those of the Nazis, then there has to be some Real Morality.
Your idea of New York may be truer than mine because New York is a real place and you may have been there whereas I have not. But when I say "New York," if I only mean the one in my head, how can it mean anything to you? Morality cannot simply be whater each happens to approve.
Lewis states that even though our different behaviors cause us to think there are different standards, the things we think about these behaviors prove otherwise.
He cautions us against equating morality with beliefs about facts. We don't kill witches because of a new standard of morality. The reason we no longer burn witches at the stake is because we no longer believe in the witches they believed in. If we, however, believed in people who sold their souls to the devil to torment their neighbors with their new supernatural powers, we would probably agree the death penalty were in order. No change in moral principle; just change in the facts. Is a man more humane because he stops setting mousetraps when he believes there are no more mice?