Can nature explain reason?

I was recently reading about C.S. Lewis’ argument against atheism in that nature alone can’t explain the fact that man has reason/rationality, because atoms/chemicals in the brain are not rational and how can rationality come from something irrational? Thus, in order for reason to exist (which it does), there has to be some kind of divine source for reason.

Keep in mind that I consider myself an atheist, or if at best, an agnostic atheist. I’m not asking this to start an atheist vs. theist debate. I’m asking this mainly toward atheists (since I know there are plenty on Fluther) and what their response to it is.