THREE: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.