Harry Cleeveley

Modal rationalism is the claim that for any proposition p, if it is ideally conceivable that p, then there is a metaphysically possible world, W, in which p is true. If true, modal rationalism must itself be an a priori truth. Moreover, modal rationalism is true just if there are no strong a posteriori necessities. But are there any strong necessities? In this paper, I set out a transcendental argument to show that there cannot be any, because they are not genuinely conceivable. I argue that if it were ideally conceivable that there are strong necessities, then it would be impossible for us to have any modal knowledge of the world. Given that we do have some modal knowledge of the world, it follows that strong necessities are not ideally conceivable and therefore that modal rationalism itself is an a priori truth.

Modal rationalism is the claim that for all p, if it is ideally conceivable that p, then there is a metaphysically possible world, W, in which p is true. This will be true just if there are no strong a posteriori necessities, where a strong necessity (for short) is a proposition that is conceivably false, but which is true in all metaphysically possible worlds. But are there any strong necessities? Various alleged examples have been proposed and argued over in the literature, but there is no consensus on whether any is genuine. In this paper, I aim to move the debate forwards by proving that there are in fact no strong necessities. I argue that there are no strong necessities because they are impossible; they are impossible because the very notion of a strong necessity is – despite prima facie appearances – ultimately incoherent. Thus, I argue, it is an a priori truth that there are no strong necessities, and that modal rationalism itself is true.