 This barrier between visible and hidden qualities/powers is independent of the more fundamental problem of induction that even when one grasps, in some way, the structure of the hidden qualities ordinarily responsible for conjunction of sensible qualities, one cannot feel secure about their presence and role in any claim one makes about sensible qualities. For, a different set of hidden qualities may be responsible for any ordinarily conjoined set of sensible qualities. (Hume here discerns the outlines of the problem of multiple realization as a general metaphysical problem.) That, in addition, nature may change its course (and so change the configuration of hidden qualities that produce sensible qualities) means that the main explanatory framework of early modern science (i) is without foundation (see also Belkind 2019).