Full Idea
The sentences that contain metaphors are typically obviously false or trivially true, because these are typically indications that something is intended as a metaphor.
Gist of Idea
We indicate use of a metaphor by its obvious falseness, or trivial truth
Source
Donald Davidson (Truth and Predication [2005], 6)
Book Reference
Davidson,Donald: 'Truth and Predication' [Belknap Harvard 2005], p.123
A Reaction
A nice point which sounds correct. Metaphors are famous being false, but the 'obvious' falseness signals the metaphor. If a metaphor is only obscurely false, that makes it difficult to read.