Full Idea
Analyticity is postulated to explain why we cannot imagine certain things being true. A better postulate is that we are not good at imagining things.
Gist of Idea
Analyticity is postulated because we can't imagine some things being true, but we may just lack imagination
Source
Gilbert Harman (Thought [1973], 6.7)
Book Reference
Harman,Gilbert: 'Thought' [Princeton 1977], p.104