178 THE THEOKi (Ji sKNSATION. 



is more fully developed by some of his followers. The difficnlty 

 arising from the reciprocal influence of matter and mind is got rid of 

 by denying that such agency exists at all. We can neither move 

 our limbs, nor derive sensations from external objects. All this is 

 done immediately by God. Certain positions of external objects are 

 only the occasibns on which God acts, so that, therefore, the presence 

 of light reflected from any substance to the optic nerve, is only the 

 occasion on which God aflTects the mind with the sensation of vision. 



Sir Isaac Newton, to account for the transfer of the brain of impres- 

 sions on the organs of sense, threw out a conjecture whether this might 

 be accomplished by a succession of vibrations of a thin elastic fluid, 

 which he called ether, and which he supposed to pervade the inter- 

 stices of all bodies. This conjecture was taken up and fulh^ developed 

 by Hartley, in his celebrated theory of vibrations. According to this 

 philosopher, the mind is two-fold, that which is percipient and active 

 being the nobler part, the other being inert and insensible, and only 

 occupied in funiishing the former with objects of thought, it being 

 the repository of the mind's ideas, which here are stored up ready for 

 use. External objects do not aff'ect the mind immediately, but only 

 by the agency of this inferior principle, the impression being con- 

 veyed from the organs to this repository, and this, in every case, 

 infoiming the active principle. " The mind," says he, " sits retired 

 in kingly state, nothing external, or bodily, being admitted to its 

 presence ; and though in sensation the notice be received from things 

 without us, they only deliver their message to the mental organs, 

 which by them is carried into the royal cabinet." The ether is the 

 principal agent in conveying this message, for being diffiised through 

 all the pores of the sensorium, external objects excite vibrations in it 

 •which act upon and excite corresponding vibrations in the infinitesi- 

 mal particles of the nervous matter, just as vibrations in the air affect 

 solid bodies. These vibrations being kept up by the uniform com- 

 pression of the ether, are transmitted to the brain, the internal parts 

 of which are instantly set m motion, vibrating in different directions, 

 according to the different directions of the nerves by which the vibra- 

 tions enter. The great argument, or rather assumption, by which 

 this curious speculation is defended, I will give in the theorist's own 

 words : — " No motion but a vibratory one can reside in any part for 

 the least moment of time, and exteraal objects being corporeal can 

 act on the nerves and brain, which are also corporeal, by nothing but 

 impressing motion on them." 



Another celebrated theory is that of Leibnitz, a philosopher of 

 Leipsic. He, like Des Cartes, endeavoured to avoid the difficulty 

 involved in the physical action of substances so different as mind and 

 matter, by denying such agency, though he accounted for the pheno- 

 mena in a manner ditTerent from that of the French philosopher. He 

 supposed the entire universe to be composed of monads, which had 

 all their powers and tendencies bestowed on them at their first 

 creation. All the events which occur in nature are only the evolution 

 of these original powers. The body is a collection of such monads, 

 and all the changes which occur in it, and all the impressions made on 



