An early statement of Lewis's in response to Russellian Naturalism
In his [Bertrand Russell’s] “Worship of a Free Man” I found a very clear and noble statement of what I myself believed a few years ago. But he does not face the real difficulty -- that our ideals are after all a natural product, facts with a relation to all other facts, and cannot survive the condemnation of the fact as a whole. The Promethean attitude would be tenable only if we were really members of some other whole outside the real whole: wh[ich] we’re not. (Saturday, 5 January, 1924; before he was a Christian)
The idea is that if you say the universe is bad, and the universe produced you and the very thought that the universe is bad, isn't your thought tainted at the source?
The link is to Jim Slagle's blog.