Doubts about a maxim like Hume’s have a variety of historical sources. Some date back as far as Descartes’s claim that, since he can conceive himself in a purely mental condition, his essence is only to think. “How does it follow,” Arnauld asks, “from the fact that he is aware of nothing else belonging to his essence, that nothing else does in fact belong to it?” Others are as recent as the discovery by Kripke and Putnam of necessary truths knowable only a posteriori: May 24, 2004 Is Conceivability a Guide to Possibility? S. Yablo
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