Since the author thought materialism was thoroughly false, this constitutes one of the mildest attacks on the character of materialists he could have written, short of just omitting the topic altogether. Thinking your opponents are wrong because they were misled by epistemic sleight of hand (from malevolent ageless invisible all-seeing schemers, no less) is nicer than thinking they're wrong due to stupidity or sheer contrarian defiance.
The Screwtape Letters is a hundred-page collection of mistakes to be avoided. It seems silly to assume hostile intent behind this particular passage, rather than reading it in the same "don't be misled in this particular way" spirit as the rest of the text.
Since the author thought materialism was thoroughly false
That's actually one of the reasons I'm more inclined to interpret it that way. If Lewis thought there were also materialists with superb reasoning abilities who believed in materialism by correctly exercising those reasoning abilities, I wouldn't think a passage about poorly reasoned materialists was meant to be a generalization.
this constitutes one of the mildest attacks on the character of materialists he could have written
Hypothesis: Lewis was well aware that there are materialists who are...
Another month has passed and here is a new rationality quotes thread. The usual rules are: