It suffices to increase or diminish in a slight degree the rapidity of the blood penetrating to the brain, in order to cause an immediate change of our 'ego.’ The equilibrium of the molecules in the organs where consciousness has its seat is greatly disturbed by causes which scarcely affect the functions of other parts of the body; because, in the brain, nutrition is more active, and the state of the substances composing it more unstable. The sublimity of psychic phenomena has its root in the greater complication of the material facts by which they are originated. If I were asked which of the functions of the organism were most sensitive to the slightest organic change, I should, without hesitation, answer--_consciousness_. VIII Often, in contemplating the brain of my patients, pondering over its structure and functions, and seeing the blood coursing through it, I have imagined that I might penetrate into the inner life of the brain-cells, might follow the movements which agitate their minute branches in the labyrinth of the nerve-centres; I have thought I might learn the laws of organic change, the order, harmony, the most perfect concatenations; but my mind might work as it listed, and imagination seize the reins, I never yet saw anything, not the faintest gleam, which gave me hope of penetrating to the source of thought. During my investigations I have discovered the mechanism with which nature provides for a more rapid circulation of blood when the brain must enter into activity; I was the first to admire some of the phenomena in which the material activity of this organ reveals itself; but although I have scrutinised the functions of the brain with the most exact methods of physiological investigation while it was pulsing under my eyes, while ideas were seething in it, or while it rested in sleep, the nature of the psychic processes still remains a mystery. We all believe that the faculties of the mind are the fruit of an uninterrupted series of natural causes, of physical and chemical actions which lead from the simplest reflex-movements, step by step, to instinct, reason, sentiment, and will; but as yet nothing has been found which might lead us even to suspect, much less to comprehend, the nature of consciousness. We attain our firmest convictions in the domain of positivism, not from the narrow field of physiology but from the whole kingdom of science. We imagine that the impressions of the external world form a current which penetrates the nerves, and, without either abatement or check, diffuses and transforms itself in the centres, finally reappearing in the sublime form of the idea; this is the notion of the soul held by the philosophers of remote antiquity; this is the base of modern psychology. We may suppose that thought must be a form of motion, because the science of the present day demonstrates that all intimately known phenomena may be reduced to a vibration of atoms and to a displacement of molecules.