88 THE POPULAR SCIENCE MONTHLY. 



metaphysical problems. In fact, the sensory operations have been, from 

 time immemorial, the battle-ground of philosophers. 



I have more than once taken occasion to point out that we are in- 

 debted to Descartes, who happened to be a physiologist as well as a 

 philosopher, for the first distinct enunciation of the essential elements 

 of the true theory of sensation. In later times, it is not to the works of 

 the philosophers, if Hartley and James Mill are excepted, but to those 

 of the physiologists, that we must turn for an adequate account of the 

 sensory process. Haller's luminous, though summary, account of sen- 

 sation in his admirable " Primse Lineae," the first edition of which was 

 printed in 1747, offers a striking contrast to the prolixity and confusion 

 of thought which pervade Reid's " Inquiry," of seventeen years' later 

 date.* Even Sir William Hamilton, learned historian and acute critic 

 as he was, not only failed to apprehend the philosophical bearing of 

 long-established physiological truths ; but, when he affirmed that there 

 is no reason to deny that the mind feels at the finger-points, and none 

 to assert that the brain is the sole organ of thought,f he showed that 

 he had not apprehended the significance of the revolution commenced, 

 two hundred years before his time, by Descartes, and effectively fol- 

 lowed up by Haller, Hartley, and Bonnet, in the middle of the last 

 century. 



In truth, the theory of sensation, except in one point, is, at the pres- 

 ent moment, very much where Hartley, led by a hint of Sir Isaac New- 

 ton's, left it, when, a hundred and twenty years since, the " Observa- 

 tions on Man: his Frame, his Duty, and his Expectations," was laid 

 before the world. The whole matter is put in a nutshell in the follow- 

 ing passages of this notable book : 



External objects impressed upon the senses occasion, first on the nerves on 

 which they are impressed, and then on the brain, vibrations of the small and, as 

 we may say, infinitesimal medullary particles. 



These vibrations are motions backward and forward of the small particles ; 



* In justice to Reid, "however, it should be stated that the chapters on Sensation in 

 the "Essays on the Intellectual Powers " (1785) exhibit a great improvement. He is, in 

 fact, in advance of his commentator, as the note to Essay II., chap, ii., p. 248 of Hamil- 

 ton's edition shows. 



\ Haller, amplifying Descartes, writes in the "Primae Lineae," cccxvi. : "Non est 

 adeo obscurum sensum omnem oriri ab objecti sensibilis impressione in nervum quem- 

 cumque corporis humani, et eamdem per cum nervum ad cerebrum pervenientem tunc 

 demum representari animae, quando cerebrum adtigit. Ut etiam hoc falsum sit auimam 

 inproximo per sensoria nervorumque ramos sentire." . . . dlvii. : " Dum ergo senti- 

 mus quinque diversissima entia conjunguntur : corpus quod sentimus : organi sensorii 

 adfectio ab eo corpore : cerebri adfectio a sensorii percussione nata : in anima nata mu- 

 tatio: anima? denique conscientia et sensationis adperceptio." Nevertheless, Sir Wil- 

 liam Hamilton gravely informs his hearers : " We have no more right to deny that the 

 mind feels at the finger-points, as consciousness assures us, than to assert that it thinks 

 exclusively in the brain." " Lecture on Metaphysics and Logic," ii., p. 128. " We have 

 no reason whatever to doubt the report of consciousness, that we actually perceive at the 

 external point of sensation, and that we perceive the material reality." Ibid., p. 129. 



