When the doctor at a post-mortem diagnoses poison, pointing to the state of the dead man's organs, his argument is rational because he has a clear idea of that opposite state in which the organs would have been found if no poison were present. In the same way, if we use the vastness of space and the smallness of earth to disprove the existence of God, we ought to have a clear idea of the sort of universe we should expect if God did exist. But have we?
Whatever space may be in itself - and, of course, some moderns think it finite - we certainly perceive it as three-dimensional, and to three-dimensional space we can conceive no boundaries. By the very forms of our perceptions, therefore, we must feel as if we lived somewhere in infinite space. If we discovered no objects in this infinite space except those which are of use to man (our own sun and moon), then this vast emptiness would certainly be used as a strong argument against the existence of God. If we discover other bodies, they must be habitable or uninhabitable: and the odd thing is that both these hypotheses are used as grounds for rejecting Christianity. If the universe is teeming with life, this, we are told, reduces to absurdity the Christian claim - or what is thought to be the Christian claim - that man is unique, and the Christian doctrine that to this one planet God came down and was incarnate for us men and for our salvation. If on the other hand, the earth is really unique, then that proves that life is only an accidental by-product in the universe, and so again disproves [the] religion.
Really, we are hard to please. We treat God as the police treat a man when he is arrested, whatever He does will be used in evidence against Him. I do not think this is due to...wickedness. I suspect that there is something in our very mode of thought which makes it inevitable that we should always be baffled by actual existence, whatever character actual existence may have.
- C.S. Lewis: God in the Dock: Essays on Theology and Ethics (Ed. Walther Hooper). Originally published as Undeceptions: Essays on Theology and Ethics in the UK