Full Idea
I have come to think that, although many things can be known to be complex, nothing can be known to be simple.
Gist of Idea
Complex things can be known, but not simple things
Source
Bertrand Russell (My Philosophical Development [1959], Ch.14)
Book Reference
Russell,Bertrand: 'My Philosophical Development' [Routledge 1993], p.123
A Reaction
This appears to be a rejection of his logical atomism. It goes with a general rebellion against foundationalist epistemology, because the empiricists foundations (e.g. Hume's impressions) seem devoid of all content.