Empirically-Informed Modal Rationalism

In this chapter, it is suggested that our epistemic access to metaphysical modality generally involves rationalist, a priori elements. However, these a priori elements are much more subtle than ‘traditional ’ modal rationalism assumes. In many cases of modal inquiry, a priori and a posteriori elements are deeply intertwined and it is not easy to tell them apart. Supposed metaphysically necessary identity statements involving natural kind terms are a good example: the fact that empirical input is crucial in establishing their necessity has clouded the role and content of the a priori input. For instance, as I have argued in previous work, the supposed metaphysically necessary identity statement involving water and its microstructure can only be established with the help of a controversial a priori principle concerning the determination of chemical properties by microstructure. The Kripke-Putnam framework of modal epistemology fails precisely because it is unclear whether the required a priori element is present. One upshot is that there may be very few substantial metaphysical necessities involving natural kinds. My positive proposal is motivated by a non-modal conception of essence, i.e., essence is ontologically prior to modality. Yet, our epistemic access to essence, it is suggested, is via metaphysical modality, e.g., via principles such as the one concerning the metaphysical necessity o