From the Problem of Pain, the chapter on The Fall of Man

Someone or something whispered that they could become as gods—that they could cease directing their lives to their Creator and taking all their delights as uncovenanted mercies, as ‘accidents’ (in the logical sense) which arose in the course of a life directed not to those delights but to the adoration of God. As a young man wants a regular allowance from his father which he can count on as his own, within which he makes his own plans (and rightly, for his father is after all a fellow creature), so they desired to be on their own, to take care for their own future, to plan for pleasure and for security, to have a meum from which, no doubt, they would pay some reasonable tribute to God in the way of time, attention, and love, but which, nevertheless, was theirs not His. They wanted, as we say, to ‘call their souls their own’. But that means to live a lie, for our souls are not, in fact, our own. They wanted some corner in the universe of which they could say to God, ‘This is our business, not yours.’ But there is no such corner. They wanted to be nouns, but they were, and eternally must be, mere adjectives. We have no idea in what particular act, or series of acts, the self-contradictory, impossible wish found expression. For all I can see, it might have concerned the literal eating of a fruit, but the question is of no consequence.

This act of self-will on the part of the creature, which constitutes an utter falseness to its true creaturely position, is the only sin that can be conceived as the Fall. For the difficulty about the first sin is that it must be very heinous, or its consequences would not be so terrible, and yet it must be something which a being free from the temptations of fallen man could conceivably have committed. The turning from God to self fulfils both conditions. It is a sin possible even to Paradisal man, because the mere existence of a self—the mere fact that we call it `me’—includes, from the first, the danger of self-idolatry. Since I am I, I must make an act of self-surrender, however small or however easy, in living to God rather than to myself. This is, if you like, the ‘weak spot’ in the very nature of creation, the risk which God apparently thinks worth taking.