Cerebral processes are of course largely hypothetical. Theory suggests their existence, and experience can verify that theory only in an indirect and imperfect manner. The addition of a physical substratum to all thinking is only a scientific expedient, a hypothesis expressing the faith that nature is mechanically intelligible even beyond the reaches of minute verification. The accompanying consciousness, on the other hand, is something intimately felt by each man in his own person; it is a portion of crude and immediate experience. That it accompanies changes in his body and in the world is not an inference for him but a datum. But when crude experience is somewhat refined and the soul, at first mingled with every image, finds that it inhabits only her private body, to whose fortunes hers are altogether wedded, we begin to imagine that we know the cosmos at large better than the spirit; for beyond the narrow limits of our own person only the material phase of things is open to our observation. To add a mental phase to every part and motion of the cosmos is then seen to be an audacious fancy. It violates all empirical analogy, for the phenomenon which feeling accompanies in crude experience is not mere material existence, but reactive organisation and docility. [Sidenote: Artifices involved in separating them.] The limits set to observation, however, render the mental and material spheres far from coincident, and even in a rough way mutually supplementary, so that human reflection has fallen into a habit of interlarding them. The world, instead of being a living body, a natural system with moral functions, has seemed to be a bisectible hybrid, half material and half mental, the clumsy conjunction of an automaton with a ghost. These phases, taken in their abstraction, as they first forced themselves on human attention, have been taken for independent and separable facts. Experience, remaining in both provinces quite sensuous and superficial, has accordingly been allowed to link this purely mental event with that purely mechanical one. The linkage is practically not deceptive, because mental transformations are indeed signs of changes in bodies; and so long as a cause is defined merely as a sign, mental and physical changes may truly be said to cause one another. But so soon as this form of augury tries to overcome its crude empiricism and to establish phenomenal laws, the mental factor has to fall out of the efficient process and be represented there by what, upon accurate examination, it is seen to be really the sign of—I mean by some physiological event.