A striking feature of Newton’s thought is the very broad reach of his empiricism, potentially extending even to immaterial substances, including God, minds, and should one exist, a non-perceiving immaterial medium. Yet Newton is also drawn to certain metaphysical principles—most notably the principle that matter cannot act where it is not—and this second, rationalist feature of his thought is most pronounced in his struggle to discover ‘gravity’s cause’. The causal problem remains vexing, for he neither invokes primary causation, nor accepts (...) action at a distance by locating active powers in matter. To the extent that he is drawn to metaphysical principles, then, the causal problem is that of discovering some non-perceiving immaterial medium. Yet Newton’s thought has a third striking feature, one with roots in the other two: he allows that substances of different kinds might simultaneously occupy the very same region of space. I elicit the implications of these three features. For Newton to insist upon all three would transform the causal question about gravity into an insoluble problem about apportioning active powers. More seriously, it would undermine his means of individuating substances, provoking what I call ‘Newton’s Substance Counting Problem’. (shrink)

The concept of electrical conductivity, or, as initially coined by Stephen Gray , ‘electrical communication’, has always been assigned an important role in the history of electrical research. Some thirty-five years after Gray's ‘electrical communication’ acquired wide attention, Priestley employed the concept of conductivity to define physical reality, thus giving a privileged position to the science he himself endeavoured to cultivate. As he argued in the introduction to The History and Present State of Electricity , ‘the electrical fluid is no (...) local, or occasional agent in the theatre of the world. Late discoveries show that its presence and effects are every where … It is not, like magnetism, confined to one kind of bodies, but every thing we know is a conductor or nonconductor of electricity’. Contemporary historians, for example, Heilbron, Home and Hackmann, link the concept of conductivity to a radical transformation of electrical research which pertained to its mode of organization and the definition of its subject-matter, and which culminated in its emergence as a distinctive branch of eighteenth-century ‘experimental philosophy’. (shrink)