LIMITS OF OUR KNOWLEDGE OF NATURE, 25 



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 sim- 

 plest structure, appeared that impassable gulf, and then the world be- 

 came 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 wdth 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 ofi'that philosopher from 

 all possibility of understanding this interaction. First, he held that 

 body and soul are two different substances, united by God's omnipo- 

 tence, 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 sim- 

 ply producing motion in the pineal gland, in such a way that the ani- 

 mal 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 impos- 

 sibility 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 causcB occasionales of the perpetually-renewed interventions of 

 Omnipotence. 



Finally, Leibnitz explained this problem on the hypothesis, origi- 

 nated, 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 theii 

 movements should be synchronous, as w^as 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 



