ioo THE POPULAR SCIENCE MONTHLY. 



simply from the roundabout way in which the vertebrate retina is de- 

 veloped. 



Thus all the higher sense-organs start from one foundation, and the 

 receptive epithelium of the eye, or of the ear, is as much modified epi- 

 dermis as is that of the nose. The structural unity of the sense-organs 

 is the morphological parallel to their identity of physiological function, 

 which, as we have seen, is to be impressed by certain modes of motion ; 

 and they are fine or coarse in proportion to the delicacy or the strength 

 of the impulses by which they are to be affected. 



In ultimate analysis, then, it appears that a sensation is the equiva- 

 lent in terms of consciousness for a mode of motion of the matter of 

 the sensorium. But, if inquiry is pushed a stage further, and the ques- 

 tion is asked, What then do we know about matter and motion? there 

 is but one reply possible. All that we know about motion is that it is 

 a name for certain changes in the relations of our visual, tactile, and 

 muscular sensations ; and all that we know about matter is that it is 

 the hypothetical substance of physical phenomena the assumption of 

 the existence of which is as pure a piece of metaphysical speculation 

 as that of the substance of mind. 



Our sensations, our pleasures, our pains, and the relations of these 

 make up the sum total of the elements of positive, unquestionable 

 knowledge. We call a large section of these sensations and their re- 

 lations matter and motion ; the rest we term mind and thinking ; and 

 experience shows that there is a certain constant order of succession 

 between some of the former and some of the latter. 



This is all that just metaphysical criticism leaves of the idols set up 

 by the spurious metaphysics of vulgar common sense. It is consistent 

 either with pure Materialism, or with pure Idealism, but it is neither. 

 For the Idealist, not content with declaring the truth that our knowl- 

 edge is limited to facts of consciousness, affirms the wholly unprovable 

 proposition that nothing exists beyond these and the substance of mind. 

 And, on the other hand, the Materialist, holding by the truth that, for 

 anything that appears to the contrarj^, material phenomena are the 

 causes of mental phenomena, asserts his unprovable dogma, that ma- 

 terial phenomena and the substance of matter are the sole primary 

 existences. 



Strike out the propositions about which neither controversialist does 

 or can know anything, and there is nothing left for them to quarrel about. 

 Make a desert of the Unknowable, and the divine Astraea of philosophic 

 peace will commence her blessed reign. Nineteenth Century. 



