One of the problems philosophers were trying to solve in the seventeenth and eighteenth centuries was the old problem of finding a rational explanation for our knowledge of natural phenomena. Such knowledge was then conceived, by most philosophers, as expressible by mathematical, functional, law-like relations, which, more than mere regularities, seemed to reflect the physical necessity of external relations between bodies and their states. Classical rationalists and empiricists alike had, however, realised that, if the ontological ground of natural necessity could be found at all, then it had to lie in a realm other than natural phenomena themselves.