For the first several chapters of Mere Christianity, Lewis argues for a generic theism, choosing to leave anything distinctly Christian out of the discussion until later so as not to turn away an audience who may be averse to hearing about Jesus.
In the Abolition of Man, he actually spends the entire book arguing for a generic theistic basis for morality. Here, he actually invents a definition for the word “Tao” and uses it to encompass the bulk of world religious (and even non-religious!) beliefs, in addition to Christianity. He can do that because of how narrow the point he is arguing. His adversary is brand or relativism that refuses to issue a judgement, positive or negative, about ANYTHING. It’s an interesting technique. I wish it could be used in other conversations without so much set up.
In this section, he discusses how it is impossible for outsiders to a religion to “get it” enough to be truly critical. Hostile, yes, but not properly critical as they don’t really know what they’re talking about.
Those who understand the spirit of the Tao and who have been led by that spirit can modify it in directions which that spirit itself demands. Only they can know what those directions are. The outsider knows nothing about the matter. His attempts at alteration, as we have seen, contradict themselves. So far from being able to harmonize discrepancies in its letter by penetration to its spirit, he merely snatches at some one precept, on which the accidents of time and place happen to have riveted his attention, and then rides it to death—for no reason that he can give.
From within the Tao itself comes the only authority to modify the Tao. This is what Confucius meant when he said ‘With those who follow a different Way it is useless to take counsel’.5 This is why Aristotle said that only those who have been well brought up can usefully study ethics: to the corrupted man, the man who stands outside the Tao, the very starting point of this science is invisible.6 He may be hostile, but he cannot be critical: he does not know what is being discussed. This is why it was also said ‘This people that knoweth not the Law is accursed’7 and ‘He that believeth not shall be damned’.8
An open mind, in questions that are not ultimate, is useful. But an open mind about the ultimate foundations either of Theoretical or of Practical Reason is idiocy. If a man’s mind is open on these things, let his mouth at least be shut. He can say nothing to the purpose. Outside the Tao there is no ground for criticizing either the Tao or anything else. In particular instances it may, no doubt, be a matter of some delicacy to decide where the legitimate internal criticism ends and the fatal external kind begins. But wherever any precept of traditional morality is simply challenged to produce its credentials, as though the burden of proof lay on it, we have taken the wrong position.
The legitimate reformer endeavours to show that the precept in question conflicts with some precept which its defenders allow to be more fundamental, or that it does not really embody the judgement of value it professes to embody. The direct frontal attack ‘Why?’—’What good does it do?’—’Who said so?’ is never permissible; not because it is harsh or offensive but because no values at all can justify themselves on that level. If you persist in that kind of trial you will destroy all values, and so destroy the bases of your own criticism as well as the thing criticized. You must not hold a pistol to the head of the Tao. Nor must we postpone obedience to a precept until its credentials have been examined. Only those who are practising the Tao will understand it. It is the well-nurtured man, the cuor gentil, and he alone, who can recognize Reason when it comes.9 It is Paul, the Pharisee, the man ‘perfect as touching the Law’ who learns where and how that Law was deficient.10
In order to avoid misunderstanding, I may add that though I myself am a Theist, and indeed a Christian, I am not here attempting any indirect argument for Theism. I am simply arguing that if we are to have values at all we must accept the ultimate platitudes of Practical Reason as having absolute validity: that any attempt, having become sceptical about these, to reintroduce value lower down on some supposedly more ‘realistic’ basis, is doomed. Whether this position implies a supernatural origin for the Tao is a question I am not here concerned with.
-C.S. Lewis, The Abolition of Man, p.59
Taking this idea a bit further, we find doubting philosophers “explaining away” anything and everything of certainty. Well, that’s all well and good, but you can’t go doing that forever.
The kind of explanation which explains things away may give us something, though at a heavy cost. But you cannot go on `explaining away’ for ever: you will find that you have explained explanation itself away. You cannot go on `seeing through5 things for ever. The whole point of seeing through something is to see something through it. It is good that the window should be transparent, because the street or garden beyond it is opaque. How if you saw through the garden too? It is no use trying to `see through’ first principles. If you see through everything, then everything is transparent. But a wholly transparent world is an invisible world. To `see through’ all things is the same as not to see.
-p.91
Lewis spends much of the book applying this idea to education. The book (which is only about 100 pages) should be required reading for all college freshman.