Freud asserts that as children become adults, their sense of right and wrong comes simply from what they have been taught by their parents, that "their parents' prohibitions and demands persist within them as a moral conscience." Eventually they introduce this whole system of rewards and punishments "unaltered into their religion."Lewis agrees that we learn the moral law, in part, form our parents and teachers, and that this helps develop our conscience. But this does not mean that the moral law is simply "a human invention." Lewis explains that our parents and teachers did not make up this law any more then they made up the multiplication tables which they also teach us. He points out that some of what our parents and teachers teach us "are mere conventions which might have been different -- we learn to keep to the left of the road, but it might just have been the rule to keep to the right -- and others of them, like mathematics, are real truths." Mores or customs change with time; morals and moral law hold firm.
The Screwtape Letters, By C.S. Lewis; Letter #8
Has no one ever told you about the law of Undulation?
Humans are amphibians -- half spirit and half animal... As spirits they belong to the eternal world, but as animals they inhabit time. This means that while their spirit can be directed to an eternal object, their bodies, passions, and imaginations are in continual change, for to be in time means to change. Their nearest approach to constancy, therefore, is undulation -- the repeated return to a level form which they repeatedly fall back, a series of troughs and peaks...
And that is where the troughs come in. You must have often wondered why the Enemy does not make more use of His power to be sensibly present to human souls in any degree He chooses and at any moment.... the creatures are to be one with Him, but yet themselves; merely to cancel them, or assimilate them, will not serve. He is prepared to do a little overriding at the beginning. He will set them off with communications of His presence which, though faint, seem great to them, with emotional sweetness, and easy conquest over temptation. But He never allows this state of affairs to last long. Sooner or later He withdraws, if not in fact, at least from their conscious experience, all those supports and incentives. He leaves the creature to stand up on its own legs-- to carry out from the will alone duties which have lost all relish. It is during such trough periods, much more than during the peak periods, that it is growing into the sort of creature He wants it to be. Hence the prayers offered in the state of dryness are those which please him best... He cannot 'tempt' to virtue as we do to vice. He wants them to learn to walk and must there fore take away His hand; and if only the will to walk is really there He is pleased even with their stumbles. Do not be deceived, Wormwood, Our cause is never more in danger then when a human, no longer desiring, but still intending, to do our Enemy's will, looks round upon a universe from which every trace of Him seems to have vanished, and asks why he has been forsaken, and still obeys.