„A tautology’s truth is certain, a proposition’s possible, a contradiction’s impossible.“

— Ludwig Wittgenstein

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„Propositions are truth-functions of elementary propositions. (An elementary proposition is a truth-function of itself.) (5)“
— Ludwig Wittgenstein
„If he will not be satisfied with truth in the form of tautology, that is to say, if he will not be content with empty husks, then he will always exchange truths for illusions.“
— Friedrich Nietzsche
„The truth of a proposition has little or nothing to do with its psychodynamics. The notion that ‘truth will prevail’ is merely a pious wish; history doesn’t show it.“
— Robert A. Heinlein