Greeves had written to Lewis asking about the degree to which we can speak, if at all, of God understanding evil in any kind of experiential way--as Greeves had put it, 'sharing' in our evil actions.
Lewis begins with an analogy (all emphases original)--
Supposing you are taking a dog on a lead past a post. You know what happens. . . . He tries to go the wrong side and gets his head looped round the post. You see that he can't do it, and therefore pull him back. You pull him back because you want to enable him to go forward. He wants exactly the same thing--namely to go forward: for that very reason he resists your pull back, or, if he is an obedient dog, yields to it reluctantly as a matter of duty which seems to him to be quite in opposition to his own will: tho' in fact it is only by yielding to you that he will ever succeed in getting where he wants.Lewis then goes back to the original question to bring his analogy home:
Now if the dog were a theologian he would regard his own will as a sin to which he was tempted, and therefore an evil: and he might go on to ask whether you understand and 'contained' his evil. If he did you could only reply 'My dear dog, if by your will you mean what you really want to do, namely, to get forward along this road, I not only understand this desire but share it. Forward is exactly where I want you to go. If by your will, on the other hand, you mean your will to pull against the collar and try to force yourself in a direction which is no use--why I understand it of course: but just because I understand it (and the whole situation, which you don't understand) I cannot possibly share it. In fact the more I sympathise with your real wish--that is, the wish to get on--the less can I sympathise (in the sense of 'share' or 'agree with') your resistance to the collar: for I see that this is actually rendering the attainment of your real wish impossible.'
I don't know if you will agree at once that this is a parallel to the situation between God and man: but I will work it out on the assumption that you do. Let us go back to the original question--whether and, if so in what sense God contains, say, my evil will--or 'understands' it. The answer is God not only understands but shares the desire which is at the root of all my evil--the desire for complete and ecstatic happiness. He made me for no other purpose than to enjoy it. But He knows, and I do not, how it can be really and permanently attained. He knows that most of my personal attempts to reach it are actually putting it further and further out of my reach. With these therefore He cannot sympathise or 'agree.'Lewis then relates his point to how we think about past sins, and then how we think about future sins (temptation).
I may always feel looking back on any past sin that in the very heart of my evil passion there was something that God approves and wants me to feel not less but more. Take a sin of Lust. The overwhelming thirst for rapture was good and even divine: it has not got to be unsaid (so to speak) and recanted. But it will never be quenched as I tried to quench it. If I refrain--if I submit to the collar and come round the right side of the lamp-post--God will be guiding me as quickly as He can to where I shall get what I really wanted all the time. It will not be very like what I now think I want: but it will be more like it than some suppose. In any case it will be the real thing, not a consolation prize or substitute. If I had it I should not need to fight against sensuality as something impure: rather I should spontaneously turn away from it as something cold, abstract, and artificial. This, I think, is how the doctrine applies to past sins.And in a final, powerful, delightful reminder--
On the other hand, when we are thinking of a sin in the future, i.e. when we are tempted, we must remember that just because God wants for us what we really want and knows the only way to get it, therefore He must, in a sense, be quite ruthless towards sin. He is not like a human authority who can be begged off or caught in an indulgent mood. The more He loves you the more determined He must be to pull you back from your way which leads nowhere into His way which leads where you want to go. Hence MacDonald's words 'The all-punishing, all-pardoning Father.' You may go the wrong way again, and again He may forgive you: as the dog's master may extricate the dog after he has tied the whole leash around the lamp-post. But there is no hope in the end of getting where you want to go except by going God's way. . . .
I think one may be quite rid of the old haunting suspicion--it raises its head in every temptation--that there is something else than God--some other country into which He forbids us to trespass--some kind of delight which He 'doesn't appreciate' or just chooses to forbid, but which would be real delight if only we were allowed to get it. The thing just isn't there. Whatever we desire is either what God is trying to give us as quickly as He can, or else a false picture of what He is trying to give us--a false picture which would not attract us for a moment if we saw the real thing.--Walter Hooper, ed., The Collected Letters of C. S. Lewis, Volume 2: Books, Broadcasts, and the War, 1931-1949 (Cambridge University Press, 2004), 122-24