138 140 Newton and "Empiricist" Philosophers experience. So a "system of the sciences" must, for him, be empirical. But what is an empirical "system of the sciences"? I argue that (ii) by the time Hume wrote the Treatise, English natural philosophy had indeed developed the tools for the construction of a hierarchical system of sciences derived empirically. The key to the construction of an empirical hierarchy of the sciences is provided by Boyle's conception of a step-wise "reduction" or "resolution" of the respective sciences to a small set of fundamental terms derived from physics. 7 Such explanatory reduction then appears in a new and much more powerful form in Newton's Principia. Finally, I show that (iii) Hume consistently presents his method in terms almost identical to those Newton uses to describe his own explanatory reductions. 8 Hume differs from Newton in claiming that all of experience can be reduced to psychological "elements" that he takes to be simpler and more general than those provided by physics. 9 6.1 WHAT IS A 'SYSTEM OF THE SCIENCES'? What does Hume mean when he speaks of a "system of the sciences"? We know from the Introduction to the Treatise that he conceives of all of the sciences as being interrelated-and, more specifically, that this interrelationship derives from the fact that all of the sciences are "dependent" on the study of human nature. As he writes: 'Tis evident, that all the sciences have a relation, greater or less, to human nature.. .. Even Mathematics, Natural Philosophy, and Natural Religion, are in for 'deduce': "to draw in a regular connected series, from one time to another. " See Baier (1991, p. 302, n. 8). 7 I will be using the term "reduction" to mean something rather different from what it is usually taken to mean in recent philosophy. I discuss this term and its meaning in the context of early modern philosophy in Section 2 below. 8 But Hume is not to be taken as a straightforward Newtonian. Hume in fact has a dual agenda with respect to Newton. On the one hand, Hume's aims and methods in the Treatise are strongly influenced by Newton. On the other hand, Hume's Treatise is deeply subversive with respect to Newtonianism. The subversive character of the Hume's philosophy with respect to Newton is a theme that has been raised in past work by Noxon (1973), Jones (1982), and Barfoot (1990, pp. 151-190). However, these works fail to recognize the profoundly Newtonian character of Hume's work. More balanced works that treat the anti-Newtonian aspects of Hume's philosophy alongside recognition of Newton'