272 on sir david brewster's supposed 



method of proof — the method which, beginning with a certain hypo- 

 thesis, and deducing the results to which it leads, concludes from the 

 harmony between these results and actual fact, that the hypothesis 

 is correct. And here again, as I intend to limit myself to a single 

 example, I shall choose the most elegant and specious that I can find. 



Many writers on vision have perplexed themselves with the enquiry : 

 why are objects seen erect, when their pictures on the retina are 

 inverted ? Sir David Brewster tells us that this is a necessary con- 

 sequence, and therefore a confirmation, of his Law of Visible Direc- 

 tion. " The phenomenon," he writes, u of an erect object from an 

 " inverted picture on the retina, which has so unnecessarily perplexed 

 " metaphysicians and physiologists, is a demonstrable corollary from 

 *' the law of visible direction for points. The only difficulty," he 

 adds, "which I have ever experienced in studying this subject, is, 

 " to discover where any difficulty lay." 



In examining this statement, I would repeat the remark previously 

 made, that the image or " phenomenon" of an object has no exist- 

 ence in absolute space, apart from the mind. No doubt, the language 

 familiarly employed in treatises on vision tends to suggest a contrary 

 idea to careless and unreflective readers ; and few philosophers are 

 at less pains to avoid phraseology liable to be misunderstood, than Sir 

 David Brewster himself. He not only at one time, tells us of an 

 image being formed in front of a wall, or behind a wall, according to 

 the circumstances of the experiment ; and, at another time, speaks of 

 images floating in the air at a distance of so many feet from the eye ; 

 but he even accuses certain images of assuming a position in space dif- 

 ferent from " their right position " But, of course, such language — 

 whatever be its meaning — cannot signify that images do ever actually 

 exist in space, apart from the mind. I do not affirm that images are 

 purely subjective states : modes of the ego considered per se, and out 

 of all relation to matter : modes in which the ego might have existed, 

 though matter had never been. Most metaphysicians take this view. 

 A different opinion, however, may be maintained. 3t may be held 

 that an image is not a purely subjective state, but is constituted by 

 the mind's immediate apprehension of the non-ego ; that it is a 

 product of two factors, the mental and the material, mysteriously 

 united with, or existing in relation to, one another. Being desirous 

 to avoid metaphysical discussion as far as possible, I shall not attempt 

 here to judge betwixt these two opposite theories. But, whether the 

 one or the other be correct ; whether an image be purely subjective, 

 or partake partly of the subjective and partly of the objective; this 



