The text treats of the so called metaphysical maxim in which David Hume formulated the relation between conceivability and possibility. According to this straightforward maxim, the imaginability of a situation constitutes a good reason for asserting its possibility. The text itself is divided into three parts. In the first part, I specify the exact nature of Hume’s key concept as positive prima facie conceivability. In the second part, attention is given to Reid’s critique of the metaphysical maxim. Thomas Reid put forward the objection that we are capable of thinking certain impossible propositions, which is in direct contradiction with the maxim under discussion. I show that Hume could defend himself against this objection in at least two ways: he could argue that Reid extends his maxim inappropriately from ideas to propositions. He could also argue that Reid understands the expression “to think” too broadly. The third part deals with contradictory concepts or, more exactly, ideas which, in Hume’s view, we have when we consider these concepts. Firstly, I formulate a paradox stemming from the disagreement of the maxim with the basic postulate of Hume’s psychology, then I show four strategies for overcoming this paradox.
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