I’m a strong believer in modus ponens, in any domain of discourse where I know the meanings of all the words!

Scott Aaronson in reply to Max Tegmark replying to Scott's review of Max's book. He goes on:

In physics, however, I only believe in “approximate modus ponens,” in the following sense: if I accept “A” and “A⇒B,” then I’ll tentatively accept “B,” but I might decide on further reflection that I meant something different by a word appearing in “B” than by the same word in “A” or “A⇒B.” And in any case, I rarely would’ve considered “A” or “A⇒B” certain, just very well-established. For both of those reasons, I can start with physics statements that I consider to be well-established, apply enough steps of “modus ponens,” and end with a statement I consider to be speculation!

(Emphasis mine.)