is no more incomprehensible in its material conditions than is the first grade of consciousness, i. e., sensation. With the first awakening of pleasure or pain, experienced on earth by some creature of the simplest structure, appeared that impassable gulf, and then the world became doubly incomprehensible.
Few subjects have been more perseveringly studied, more written about, or more hotly disputed, than that of the connection between body and soul in man. All the philosophical schools, as also the fathers of the Church, have had their own opinions upon this matter. The more recent philosophy is less concerned with this question; but its beginnings in the seventeenth century abounded in theories of the interaction of matter and mind.
Two hypotheses set up by Descartes shut off that philosopher from all possibility of understanding this interaction. First, he held that body and soul are two different substances, united by God's omnipotence, and that, since the soul has no extension, they can come into contact only at one point, to wit, in the so-called pineal gland of the brain. He held, secondly, that the quantity of motion in the universe is constant. The more clearly it seems to follow from this that the soul cannot produce motion in matter, the more amazed are we on seeing Descartes, in order to save free-will, represent the soul as simply producing motion in the pineal gland, in such a way that the animal spirits, or, as we would say, the nervous principle, may flow out to the appropriate muscles. Conversely, the animal spirits, excited by sense-impressions, give motion to the pineal gland, and then the soul, which is in association with the latter, notes the motion.
Descartes's immediate followers, Clauberg, Malebranche, Geulincx, endeavored to correct this patent error. They insist upon the impossibility of interaction between mind and matter, as being two distinct substances. But, in order to understand how the soul nevertheless moves the body, and is moved by it, they suppose that the soul's willing is the occasion for God each time moving the body in harmony with the soul's desire. Conversely, sense-impressions give occasion to God to modify the soul in conformity with themselves. The causa efficiens, therefore, of the changes in the body wrought by the soul, and vice versa, is always God, and the soul's willing and the sense-impressions are but the causæ occasionales of the perpetually-renewed interventions of Omnipotence.
Finally, Leibnitz explained this problem on the hypothesis, originated, as it would appear, by Geulincx, of body and soul resembling two watches, with synchronous movement. This, says he, may occur in three ways: 1. The two watches might so influence one another by means of oscillations, conveyed to a common attachment, that their movements should be synchronous, as was observed by Huyghens, and as was exemplified, in the beginning of the present century, by Breguet, in a contrivance for rendering the action of two watches more