If everything is to be cognizable through mathematics, then everything must take place necessarily; even the thoughts, resolutions, and actions of man cannot be free in the sense that they might have happened otherwise. Thus there is an evident methodological motive at work for the extension of mechanism to all becoming, even spiritual becoming. But there are metaphysical reasons also. Descartes had naïvely solved the anthropological problem by the answer that the interaction of mind and body is incomprehensible but actual. The occasionalists had hesitatingly questioned these conclusions a little, the incomprehensibility as well as the actuality, only at last to leave them intact. For the explanation that there is a real influence of body on mind and _vice versa_, though not an immediate but an occasional one, one mediated by the divine will, is scarcely more than a confession that the matter is inexplicable. Spinoza, who admits neither the incognizability of anything real, nor any supernatural interferences, roundly denies both. There is no intercourse between body and soul; yet that which is erroneously considered such is both actually present and explicable. The assumed interaction is as unnecessary as it is impossible. Body and soul do not need to act on one another, because they are not two in kind at all, but constitute one being which may be looked at from two different sides. This is called body when considered under its attribute of extension, and spirit when considered under its attribute of thought. It is quite impossible for two substances to affect each other, because by their reciprocal influence, nay, by their very duality, they would lose their independence, and, with this, their substantiality. There is no plurality of substances, but only one, the infinite, the divine substance. Here we reach the center of the system. There is but one becoming and but one independent, substantial being. Material and spiritual becoming form merely the two sides of one and the same necessary world-process; particular extended beings and particular thinking beings are nothing but the changeable and transitory states _(modi)_ of the enduring, eternal, unified world-ground. "Necessity in becoming and unity of being," mechanism and pantheism--these are the controlling conceptions in Spinoza's doctrine. Multiplicity, the self-dependence of particular things, free choice, ends, development, all this is illusion and error.