Ah, one thing? Not the Most Important Thing or any such, but one thing that I definitely learned from Lewis? Here's one from the book that was I think his own second favorite after
Perelandra, namely
The Abolition of Man:
There is no way to derive values from logic. Philosophies that deny the existence of moral absolutes (which is not the same thing, by the way, as saying that our moral perceptions are perfect or complete) must either (a) be inconsistent, (b) imply the denial of any and all morality whatsoever, (c) embrace an avowedly arbitrary code.
This insight is of continual use to me. Pragmatists, evangelical atheists, etc. who condemn the idea of transcendent or absolute morality as noxious are really saying that some moralities are
better than others---which of course implies an overarching moral standard (such as species survival) by which to judge moral codes, i.e., an absolute morality. So either one admits an absolute morality at some level, or abandons the possibility of interpersonal moral appeal.
Amazingly, some of the hardcore neo-atheist philosophers have actually got to option (c) above, explicitly defending a moral code acknowledge to be arbitrary. Daniel Dennett, for example, while arguing that morality is arbitrary, insists that "We are not going to tolerate infanticide." Why not? Because we mustn't. And
how will we stick absolutely to things that we believe are not absolute? "We can defend a deep resistance to mucking with the boundaries," Dennett says: "We could have a rational policy not even to think about certain things." So a philosopher who scorns religious believers for their blind faith ends up advocating a pure fideism or arbitrarism: we must "not even . . . think about certain things." Pure lunacy, of course, and it doesn't have a snowflake's chance of Hell of surviving in the real world.
(The quotes from Dennett above are from a well-written Wired article on the new atheists:
http://www.wired.com/wired/archive/14.11/atheism.html )
Lewis exposed it all long ago, masterfully. There are at least a couple of schmutzels that even I will never step in. Thank you, Jack.
Larry