I implore you to remember the Irishman and his two stoves.* There are progressions in which the last step is... incommensurable with the other - and in which to go the whole way is to undo all the labour of your previous journey. To reduce [objective good and evil] to a mere natural product is a step of that kind. Up to that point, 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 through' things for ever. The whole point of seeing through soemthing 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. - C.S.Lewis, in The Abolition of Man
* I confess I've no idea to which doubly-bestoved Irishman he refers. Doubtless it's delightfully pertinent.