Full Idea
The more correct the subsumption of objects from one point of view, and the greater and more glaring the incongruity from another point of view, the greater is the ludicrous effect which is produced by this contrast.
Gist of Idea
Absurdity is incongruity between correct and false points of view
Source
Arthur Schopenhauer (The World as Will and Idea [1819], I 013), quoted by Roger Scruton - Laughter §5
Book Reference
Scruton,Roger: 'The Aesthetic Understanding' [Methuen 1983], p.156
A Reaction
This accounts for ludicrous humour, but there seem to be plenty of other types. Exceptional stupidity is usually amusing without necessarily being incongruous. Though it is a departure from the sensible norm.