Years ago, a friend and I were talking about something, and he mentioned Hitler. I declared

I don't know why you ever liked that guy!

in reply to which he barked

Oh! That is a lie![1]

Well, no, it wasn't a lie. I escalated by betting him dinner on the matter. Then I explained to him that, since the truth of a proposition is a precondition for it to be known, one of the ways that I could not know why he'd ever liked Hitler would be if he'd never liked Hitler. Another way would be if I'd never believed that he'd liked Hitler, regardless of how my friend really felt about Hitler.

Indeed, the contradiction of I don't know why you ever liked that guy! is I know why you at some time liked that guy! Formally,[2] So,

I don't know why you ever liked that guy!

was a truth (though perhaps not a simple truth, as he'd had trouble seeing it).

Having won the wager, I waived the prize; my objectives in betting had all been met. Now, had he won the wager, then I'm sure that he'd have collected; but had I claimed, as he'd thought, that he'd once liked Hitler, then he'd have been quite justified in extracting the dinner; it would have disincentivized my insulting him in such a way, and off-set the felt sting of the calumny.

---

[1] That was how he spoke. He often began with Oh!, and when learning English in Hong Kong he had been taught to avoid contractions.

[2] (2015:09/24): I have edited the formal expression, seeking to have it capture more completely the structure of the natural-language expression.

Tags: logic

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