"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." Mere Christianity

What Lewis is suggesting is that this sense of right and wrong, which all people in all times and all places have had, points to another reality beyond ourselves--a supernatural reality.  We cannot reduce the moral law to simply being a fact of human behavior--because we don't behave according to it.  Nor can we say that the moral law is merely a quirk of human imagination.  If that was so then how could we say that there was anything wrong with Hitler gassing 6 million Jews?  Nor can we say that our ideas about morality are constructed by society simply because we find these patterns of behavior convenient.  Our sense of right and wrong isn't convenient sometimes--like when we want to make love to someone in a certain situation and the moral law tells us not to.  Thus, the only alternative is to believe that the moral law is a reality outside of us--outside humanity altogether--but a reality which presses in on us, seeking to make us conform to a certain standard.

To some, this pointer to a supernatural reality comes as the fragrance of life.  To others it is the smell of death.  Some seek to argue it away.  Others welcome it with open arms.  But perhaps most of us live somewhere between these two extremes.  Most of us want to argue for the moral law when it is convenient to our own ends.  And we want to deny the reality of the moral law whenever we find its demands uncomfortable.

However, as Lewis points out, this simply will not do.  The moral law is there, outside of us, and it is what it is whether we like it or not.  That being the case, does it not then make sense, for those of us who believe in Christ, to pray:

"O God, our Father, from whom the whole family in heaven and on earth derives its name, bestow upon us, at the beginning of each day, that grace which shall keep us in the fellowship of the Christian way.  Grant unto each one of us that heavenly guidance and control, in all our labors, pleasures, and trials, which will maintain our hearts in peace with one another and with you.  Graciously help and prosper us in the doing of our various duties with a willing and cheerful mind; and defend us all, by your almighty power, both from inward evil and from outward harm, so that when the day is ended it may not leave us in sorrow, strife, or shame, but in true unity and thankful rest, through your merciful favor and your forgiving love.  This we pray in Christ Jesus our Lord.  Amen." Adapted from the Book of Common Worship