 Yet Spinoza had substantial disagreements with Descartes over a wide range of metaphysical issues that bear on physical theory. Most obviously, he rejected Descartes' dualism of extended and mental substances in favor of substance monism, and correspondingly rejected Cartesian mind-body interactionism. Some of these metaphysical disagreements penetrate right to the grounds of the physical views the two thinkers share, revealing much of their agreement to be quite superficial. Consider, for example, their shared rejection of appeals to final causes in physics. Descartes held that final causal or teleological thinking is useless in physics, not because physical nature is not in fact teleological, but because our finite understanding cannot hope to understand the divine will, hence cannot grasp the purposes with which physical nature is imbued. For Spinoza, in contrast, the problem is not epistemological but metaphysical. The divine cause of the world has no will, and does not create things with a plan in mind (1p32c, p33d, s2); hence nature is simply not a teleological system at all.