We have now to consider how Aristotle treats psychology, not in connexion with biology, but as a distinct science—a separation not quite consistent with his own definition of soul, but forced on him by the traditions of Greek philosophy and by the nature of things. Here the fundamental antithesis assumes a three-fold form. First the theoretical activity of mind is distinguished from its practical activity; the one being exercised on things which cannot, the other on things which can, be changed. Again, a similar distinction prevails within the special province of each. Where truth is the object, knowledge stands opposed to sense; where good is sought, reason rises superior to passion. The one antithesis had been introduced into philosophy by the early physicists, the other by Socrates. They were confounded in the psychology of Plato, and Aristotle had the merit of separating them once more. Yet even he preserves a certain artificial parallelism between them by using the common name Nous, or reason, to denote the controlling member in each. To make his anthropology still more complex, there is a third antithesis to be taken into account, that between the individual and the community, which also sometimes slides into a partial coincidence with the other two. Aristotle’s treatise on the soul is mainly devoted to a description of the theoretical faculties—sense, and thought or reason. By sense we become acquainted with the material qualities of things; by thought with their forms or ideas. It has been already mentioned that, according to our philosopher, the organism is a system of contrary forces held in equilibrium by the soul, whose seat he supposes to be in the heart. We now learn that every sensation is a disturbance of this equilibrium. In other words, the sensorium being virtually any and every mode of matter, is raised from possibility to actuality by the presence of some one force, such as heat or cold, in sufficient strength to incline the balance that way. Here we have, quite in Aristotle’s usual style, a description instead of an explanation. The atomic notion of thin films thrown off from the object of sense, and falling on the organs of sight or touch, was but a crude guess; still it has more affinity with the discoveries of a Young or a Helmholtz than scholastic phrases about potentiality and actuality. That sensation implies a disturbance of equilibrium is, indeed, an important truth; only, the equilibrium must be conceived as a balance, not of possible sensations, but of molecular states; that is to say, it must be interpreted according to the atomic theory.