Originally posted by rwingett
I think we can say that the conclusion P (with P being 'bigfoot lives on Mars' is true. This means that not-P is necessarily false. Isn't that right, Dr.S?
"P implies NOT-(NOT-P)" is a tautology. This means that for any proposition P, P being true necessarily entails that the negation of P is false. Accepting this, one should not psychologically endorse P but refuse to deny NOT-P.
In this specific instance, you are correct. "Bigfoot lives" being true does in fact entail that "Bigfoot does not live" is false.
However, as far as I know, you are still for some inexplicable reason woefully confused and fail to endorse the formally equivalent entailment in the case where P is taken to be "God does not exist" rather than "Bigfoot lives." Why you think the actual propositional content is relevant, when a tautology (and a very elementary one at that) is what is governing the crux of the matter, has always perplexed me.