Supposing there was no intelligence behind the universe, no creative mind. In that case, nobody designed my brain for the purpose of thinking. It is merely that when the atoms inside my skull happen, for physical or chemical reasons, to arrange themselves in a certain way, this gives me, as a by-product, the sensation I call thought. But, if so, how can I trust my own thinking to be true? It's like upsetting a milk jug and hoping that the way it splashes itself will give you a map of London. But if I can't trust my own thinking, of course I can't trust the arguments leading to Atheism, and therefore have no reason to be an Atheist, or anything else. Unless I believe in God, I cannot believe in thought: so I can never use thought to disbelieve in God.
-C.S. Lewis, The Case for Christianity, p 32.
We can formalise Lewis' argument like this:
- Lewis cannot imagine how we can reason unless we were designed by God
- We can reason
- Therefore we were designed by God
Personally, I think his reasoning is flawed. A lack of knowledge or understanding is a very poor start to an argument. This is an argument from ignorance. We do not know, therefore I get to inset my pet theory as the default.
The reality is that we trust our reasoning because it is so successful. We learn to reason from a very early age. Babies reason that a ball put in a box will still be there when he looks in the box again. he has reasoned that the ball should still be there, and experience has taught him that reasoning works.
Lewis, however, has this all backwards. In Lewis' world, he starts from God existing. As God exists, says Lewis, then I can be sure my reasoning is sound.
It could be argued that both arguments a circular. This is a fundamental problem. If you are using reasoning to prove that reasoning is sound, there is some circularity present.
However, Lewis' argument is further flawed because it makes two assumptions. He is assuming God exists and he is assuming that if there is a God he would make us capable of reasoning. Why should we believe either is true? Indeed, this quote comes from a book where Lewis is trying to show the Christian God exists, and to do that, he is obliged to assume that the Christian God exists!