Let us take the second [argument] first. And let us strip it of the illegitimate emotional power it derives from the word ‘stagnation’ with its suggestion of puddles and mantled pools.
If water stands too long it stinks. To infer thence that whatever stands long must be unwholesome is to be the victim of metaphor. Space does not stink because it has preserved its three dimensions from the beginning. The square on the hypotenuse has not gone moldy by continuing to equal the sum of the squares on the other two sides. Love is not dishonored by constancy, and when we wash our hands we are seeking stagnation and “putting the clock back,” artificially restoring our hands to the status quo in which they began the day and resisting the natural trend of events which would increase their dirtiness steadily from our birth to our death.
For the emotive term ‘stagnant’ let us substitute the descriptive term ‘permanent.’ Does a permanent moral standard preclude progress? On the contrary, except on the supposition of a changeless standard, progress is impossible. If good is a fixed point, it is at least possible that we should get nearer and nearer to it; but if the terminus is as mobile as the train, how can the train progress towards it? Our ideas of the good may change, but they cannot change either for the better or the worse if there is no absolute and immutable good to which they can recede. We can go on getting a sum more and more nearly right only if the one perfectly right is “stagnant”.
And yet it will be said, I have just admitted that our ideas of good may improve. How is this to be reconciled with the view that “traditional morality” is a depositum fidei which cannot be deserted? The answer can be understood if we compare a real moral advance with a mere innovation. From the Stoic and Confucian, “Do not do to others what you would not like them to do to you”; to the Christian, “Do as you would be done by” is a real advance. The morality of Nietzsche is a mere innovation.
The first is an advance because no one who did not admit the validity of the old maxim could see reason for accepting the new one, and anyone who accepted the old would at once recognize the new as an extension of the same principle. If he rejected it, he would have to reject it as a superfluity, something that went too far, not as something simply heterogeneous from his own ideas of value.
But the Nietzschean ethic can be accepted only if we are ready to scrap traditional morals as a mere error and then to put ourselves in a position where we can find no ground for any value judgements at all. It is the difference between a man who says to us: “You like your vegetables moderately fresh; why not grow your own and have them perfectly fresh?” and a man who says, “Throw away that loaf and try eating bricks and centipedes instead.”
Real moral advances, in fine, are made from within the existing moral tradition and in the spirit of that tradition and can be understood only in the light of that tradition. The outsider who has rejected the tradition cannot judge them. He has, as Aristotle said, no arche, no premises.
(Extra line breaks added for readability.) Source: The Poison of Subjectivism, in which I am reminded how much I like C. S. Lewis' writings.