The following quote has been making the rounds on the atheism tag.  Unfortunately, I haven’t seen anyone refute it.  It’s a highly refutable quote.  Let’s consider the quote:

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 point of the argument can be found in the last two sentences.  Basically put, the argument states that thought comes from god; therefore, one must believe in god in order to trust their thoughts.  What Lewis does in this argument, in an underhanded manner, is shift the burden of proof.  He presupposes and asserts the existence of his god without providing evidence.  Therefore, upon removing his presupposition, the argument works against him and any Christian who shares his reasoning:

But if I can’t trust my own thinking, of course I can’t trust the arguments leading to Christianity, and therefore have no reason to be a Christian, or anything else.

He must provide evidence for his god in order for his argument to work.  He didn’t do so; no Christian has for that matter.  Lewis discarded atheism without justification and simply asserted that he can trust his thoughts because he believes in god — a god that he didn’t provide evidence for from the onset.  In doing that, he is attempting to shift the burden of proof onto the atheist; he’s attempting to get atheists to disprove his presupposition since that would be required to disprove his argument.  Unfortunately, he committed a slight-of-hand logical fallacy and that’s where the argument fails.