18 PHYSIOLOGY. 



gives occasion to new motions in the nervous system. This 

 mutual communication or influence we assume with confidence 

 as a fact ; but the mode of it we do not understand, nor pretend 

 to explain ; and therefore we are not bound to obviate the diffi- 

 culties that attend any of the suppositions which have been made 

 concerning it. 



" I have added, ' of the mind alone f but there is some little 

 ambiguity with respect to this, therefore I would take it as at 

 first 'of the mind,' and say that thought must have for its cause 

 an immaterial substance, and that, at least ultimately, every 

 phenomenon of thinking is to be referred thereto. A demon- 

 stration of this proposition would lead me into a very long di- 

 gression, which is not much connected with this subject, and 

 the discussion might appear subtile and difficult. I am con- 

 vinced that this proposition has been demonstrated by divines 

 and metaphysicians, to whom therefore I must trust, and refer 

 you for farther satisfaction ; and I shall have occasion immedi- 

 ately to refer you to a physician for the proofs of it. But while 

 we agree with mankind in general, and with philosophers, with 

 regard to this fact, we observe, in the next place, that * this im- 

 material and thinking part of man is so connected with the ma- 

 terial and corporeal part of him, and particularly with the 

 nervous system, that motions excited in this give occasion to 

 thought ; and thought, however occasioned, gives occasion to 

 new motions in the nervous system/ I need not illustrate this 

 further, as it is so universally understood and agreed upon in every 

 system that thinking or thought arises from certain changes in 

 the state of the body ; and it is as evident that certain modifi- 

 cations of thought, however they arise, do produce motions in 

 the material or corporeal part ; and I say that ' this mutual 

 communication or influence we assume with confidence as a fact.' 

 There is not in nature, seemingly in the acknowledgment of all 

 philosophers, a greater mystery than this mutual action of the 

 soul and body upon one another ; philosophers have at least 

 talked about the matter, and there are three very celebrated sys- 

 tems with regard to it. 



" The system of Aristotle, which is adopted by many moderns, 

 is called the system of the physical influx. Des Cartes has pro- 

 posed another, which is known by the name of the system 



