 This point can be alternatively expressed. A difficult problem in giving an account of intrasubstantial causation concerns the explanation of the difference between causation among systems and causation within a system. For instance, do the works of a watch constitute an isolated system? For even Rolexes, constructed for deep-sea divers, are affected at extreme temperatures and depths. So there is a problem giving an account of causation within a Rolex. A similar problem arises for a seventeenth century philosopher who holds a mechanistic and materialist notion of substance. But Leibniz has no problem with explaining the difference between systems that are causally isolated and systems that cannot be made absolute; his monadic ontology is readily equipped to handle causally isolated units or unities. For his substances are “windowless,” not at all causally compromised by external substances.