Full Idea
Every proposition which we can understand must be composed wholly of constituents with which we are acquainted.
Gist of Idea
Every understood proposition is composed of constituents with which we are acquainted
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 6)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.32
A Reaction
This is somewhere between Hume and logical positivism, but it concerns understanding (not meaning) of propositions (not sentences), and its acquaintance can be of universals as well as of sense experience. I like Russell's version more than Ayer's.