472 FALLACIES. 



riably taking for granted that the properties of the elements must re- 

 semble those of the compounds which were formed from them. 



To descend to more modern instances ; it was long thought and was 

 stoutly maintained by the Cartesians and even by Leibnitz against the 

 Newtonian philosophy, (nor did Newton himself, as we have seen, 

 contest the assumption, but eluded it by an arbitrary hypothesis,) that 

 nothing (of a physical nature at least) could account for motion, except 

 previous motion ; the impulse or impact of some other body. It was 

 very long before the scientific world could prevail upon itself to admit 

 attraction and repulsion [I.e. spontaneous tendencies of particles to ap- 

 proach or recede from one another) as ultimate laws, no more requiring 

 to be accounted for than impulse itself, if indeed the latter were not, 

 in truth, resolvable into the former. From this same source arose the 

 innumerable hypotheses to explain those classes of motions which ap- 

 peared more mysterious than others because there was no obvious 

 mode of attributing them to impulse, as for example the voluntary mo- 

 tions of the human body. Such were the interminable systems of 

 vibrations propagated along the nerves, or animal spirits rushing up 

 and down between the muscles and the brain : which, if the facts 

 could have been proved, would 710 doubt have been an important ad- 

 dition to our knowledge of physiological laws ; but the mere invention, 

 or arbitrary supposition of them, could not unless by the strongest de- 

 lusion be supposed to render the phenomena of animal life more com- 

 prehensible or less mysterious. Nothing, however, seemed satisfacto- 

 ry, but to make out that motion was caused by motion ; by something 

 like itself. If it was not one kind of motion it must be another. In 

 like manner it was supposed that the physical qualities of objects 

 must arise from some similar quality, or perhaps only some quality 

 bearing the same name, in the particles or atoms of which the objects 

 were composed ; that a sharp taste, for example, must arise from shai-p 

 particles. And reversing the inference, the effects produced by a 

 phenomenon must, it was supposed, resemble in their physical attri- 

 butes the phenomenon itself. The influences of the planets were sup- 

 posed to be analogous to their visible peculiarities : Mars, being of a 

 red color, portended fire and slaughter ; and the like. 



Passing from physics to metaphysics, we may notice among the most 

 remarkable fruits of this a priori fallacy, two closely analogous theories, 

 employed in ancient and in modern times to bridge over the chasm 

 between the world of mind and that of matter: the species sensihiles of 

 the Epicureans, and the modern doctrine of perception by means of 

 ideas. These theories are indeed, probably, indebted for their exist- 

 ence not solely to the fallacy in question, but to that fallacy combined 

 with another natural prejudice already adverted to, that a thing cannot 

 act where it is not. In both doctrines it is assumed that the phenom- 

 enon which takes place in us when we see or touch an object, and 

 which we regard as an effect of that object, or rather of its presence to 

 our organs, must of necessity resemble very closely the outward object 

 itself To fulfill this condition, the Epicureans supposed that objects 

 were constantly projecting in all directions impalpable images of them- 

 selves, which entered at the eyes and penetrated to the mind : while 

 modem philosophers, though they rejected this hypothesis, agi'eed in 

 deeming it necessary to suppose that not the object itself, but a mental 

 image or representation of it, was the direct object of perception. Dr. 



