Full Idea
It is only when a sentence is taken to be false that we accept it as a metaphor.
Gist of Idea
We accept a metaphor when we see the sentence is false
Source
Donald Davidson (What Metaphors Mean [1978], p.40)
Book Reference
'On Metaphor', ed/tr. Sacks,Sheldon [Chicago 1981], p.40
A Reaction
This strikes me as a very nice and true generalisation, even though Davidson mentions "no man is an island" as a counterexample. We thirst for meaning, and switch to a second meaning when the first one looks peculiar.