FALLACIES OF SIMPLE INSPECTION. 335 



very long before the scientific world could prevail upon itself 

 to admit attraction and repulsion (i. e. spontaneous tendencies 

 of particles to approach or recede from one another) as ulti 

 mate laws, no more requiring to be accounted for than impulse 

 itself, if indeed the latter were not, in truth, resolvable into 

 the former. From the same source arose the innumerable 

 hypotheses devised to explain those classes of motions which 

 appeared more mysterious than others because there was no 

 obvious mode of attributing them to impulse, as for example 

 the voluntary motions 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 have been an important addition to our know 

 ledge of physiological laws ; but the mere invention, or arbi 

 trary supposition of them, could not unless by the strongest 

 delusion be supposed to render the phenomena of animal life 

 more comprehensible, or less mysterious. Nothing, however, 

 seemed satisfactory, 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 sup 

 posed 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 

 sharp particles. And reversing the inference, the effects pro 

 duced by a phenomenon must, it was supposed, resemble in 

 their physical attributes the phenomenon itself. The influences 

 of the planets were supposed to be analogous to their visible 

 peculiarities : Mars, being of a red colour, 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 modern 

 times to bridge over the chasm between the world of mind 

 and that of matter : the species sensibiles of the Epicureans, 

 and the modern doctrine of perception by means of ideas. 

 These theories are indeed, probably, indebted for their exis- 



