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Full Idea
The phenomenon of understanding sentences one has never encountered before is decisive against theories of meaning which do not proceed compositionally.
Clarification
'Compositionality' is building up meanings a bit at a time
Gist of Idea
Encountering novel sentences shows conclusively that meaning must be compositional
Source
Christopher Peacocke (Truly Understood [2008], 4.3)
Book Ref
Peacocke,Christopher: 'Truly Understood' [OUP 2008], p.135
A Reaction
I agree entirely. It seems obvious, as soon as you begin to slowly construct a long and unusual sentence, and follow the mental processes of the listener.