Fifteen centuries after Aristotle, we reach what may be called the modern settlement of the relations of mind and body, effected by Thomas Aquinas. He extended the domain of the independent immaterial principle from the highest intellectual soul of Aristotle to all the three souls recognized by him--the vegetable or plant soul (without consciousness), the animal soul (with consciousness), and the intellect throughout. The two lower souls--the vegetable and the animal--need the coöperation of the body in this life; the intellect works without any bodily organ, except that it makes use of the perceptions of the senses. A. _Vegetable or Nutritive Soul._ ---- Incorporates an immaterial part, although unconscious. .... B. _Animal Soul._ ---- Has an immaterial part, with consciousness. .... C. _Intellect._ .... Purely immaterial. The animal soul, B, contains sensation, appetite, and emotion, and is a mixed or two-sided entity; but the intellect, C, is a purely one-sided entity, the immaterial. This does not relieve our perplexities; the phenomena are still generically allied and continuous--sensation passes into intellect without any breach of continuity; but as regards the agencies, the transition from a mixed or united material and immaterial substance to an immaterial substance apart, is a transition to a differently constituted world, to a transcendental sphere of existence. The settlement of Aquinas governed all the schools and all the religious creeds, until quite recent times; it is, for example, substantially the view of Bishop Butler. At the instance of modern physiology, however, it has undergone modifications. The dependence of purely intellectual operations, as memory, upon the material processes, has been reluctantly admitted by the partisans of an immaterial principle; an admission incompatible with the isolation of the intellect in Aristotle and in Aquinas. This more thorough-going connection of the mental and the physical has led to a new form of expressing the relationship, which is nearer the truth, without being, in my judgment, quite accurate. It is now often said _the mind and the body act upon each other_; that neither is allowed, so to speak, to pursue its course alone--there is a constant interference, a mutual influence between the two. This view is liable to the following objections: 1. In the first place, it assumes that we are entitled to speak of mind apart from body, and to affirm its powers and properties in that separate capacity. But of mind apart from body we have no direct experience, and absolutely no knowledge. The wind may act upon the sea, and the waves may react upon the wind; but the agents are known in separation--they are seen to exist apart before the shock of collision; but we are not permitted to see a mind acting apart from its material companion.