I tripped over a phrase in a book similar to "Once you stop to think about it, the existence of rainbows is pretty amazing."
Now, in context, it is pretty clear that I am supposed to stop my usual thoughtless existence to take some time thinking about rainbows, but literally, the phrase could mean that I should stop all this useless thinking to have time to appreciate rainbows more.
Is the phrase "stop to think about it" actually ambiguous or is it always used in the sense of thinking?
Best Answer
No, it's not ambiguous. It's an idiom, and it has exactly the sense that you said was "pretty clear".
The thing is, stop only takes gerund complement clauses, and does not take infinitives.
When an infinitive clause occurs after stop -- in exactly the place it would if it were a complement (thank you very much, English syntax) -- it is not a complement (i.e, it's not what you're supposed to stop doing). Rather, it's a purpose infinitive (i.e, it's an adverbial clause explaining the purpose of your stopping).
You can tell the difference if you substitute an adverbial phrase for the infinitive.
means
The idiom I mentioned in the first line is the once NP VP construction, as in
The VP has to be an event; pure stative predicates are interpreted as change-of-state events, which are called Inchoatives /ɪn'kowətɪvz/ in the trade; it's a great word to throw into a cocktail party conversation.
... and, like all idioms, it sounds very weird after you repeat it a lot. Welcome to syntax.