 It is far from clear that any thorough and consistent account of Spinoza's physical theory can be found. He says too little that is focused and direct, and the various partial and indirect discussions of such fundamental topics as inertia and the individuation of bodies are individually underdeveloped and problematic, as well as in prima facie tension with one another. A few general interpretive strategies present themselves. We might take both his claim that all determinations of extended modes of substance are extrinsically caused by other finite modes and his talk of essence and conatus seriously. On such a reading, Spinoza tried to develop an ecumenical account of bodies that both conformed to the mechanical principles of the Cartesian view and preserved the sense that bodies are centers of real activity; but his entitlement to that sense is hard to square with mechanism and only dubiously earned through the appeal to essences. We might, on the other hand, focus on either of the two aspects of Spinoza's thinking about the physical, downplaying the other. Focusing on conatus and essence might enable us to take the latter, properly ethical and psychological half of the Ethics at face value, but only at the cost of being debarred from seeing how it fits with the mechanical view of the physical world, and those field metaphysical refinements of it that form the basis of modern physics. Focusing on the idea of wholly extrinsic modal determinism, we will see Spinoza as a visionary thinker whose physical theory both anticipates and provides a metaphysical basis for contemporary physical views, albeit without their central experimental and perhaps quantitative dimensions; but then the ethical and psychological half of Spinoza's thought, depending as it does on the idea that individuals, including bodies, are centers of active striving, proper to themselves, is cut adrift. Or we might take each aspect seriously for what they are worth in their own domains, seeing Spinoza as both a visionary thinker about the physical and a subtle and original theorist of the psychological, but one whose doctrines cannot be squared with each other, or with the sort of naturalism that sees the acting subject as fitting seamlessly into physical nature. But in any of these cases, we lose the unity of Spinoza's thought, which was clearly of vital importance to him.