Basis of knowledge—Perception—Constitution of brain—White and grey matter—Average size and weight of brains—European, negro, and ape—Mechanism of perception—Sensory and motor nerves—Separate areas of brain—Sensory and motor centres—Abnormal states of brain—Hypnotism—Somnambulism—Trance—Thought-reading—Spiritualism—Reflex action—Ideas how formed—Number and space—Creation unknowable—Conceptions based on perceptions—Metaphysics—Descartes, Kant, Berkeley—Anthropomorphism—Laws of nature. Before entering on the higher subjects of religions and philosophies, it is well to arrive at some precise idea of the limits of human knowledge, and of the boundary line which separates the knowable from the unknowable. The ultimate basis of all knowledge is perception. Without an environment to create impressions, and an organ to receive them, we should know absolutely nothing. What is the environment and what the organ of human knowledge? The environment is the whole surrounding universe, or, in the last analysis, the motions, or changes of motion, by which the objects in that universe make impressions on the recipient organ. The organ is the grey matter of that large nervous agglomeration, the brain. But here I must at the outset make two reservations. In the first place I do not define how these impressions are made. In all ordinary cases they are made through the channels of the senses; but it is possible that in certain exceptional cases vibrations in the brain, causing perceptions, may be conveyed to it through the nerves in other ways. In somnambulism, for instance, it seems to be an ascertained fact that a somnambulist with closed eyes securely bandaged can walk in the dark and avoid obstacles as well as if guided by the sight in full daylight. There is a great deal of evidence also that in artificial somnambulism, otherwise called mesmerism or hypnotism, and also in what is called thought-reading, perceptions may be conveyed from one brain to another otherwise than by the usual methods of speech or writing. But these phenomena, however far they may be extended, do not affect the position that impressions on the brain are the essential condition of thought. If the grey matter of the brain is deficient or diseased the mind is affected, and beyond a certain point becomes extinct.