FALLACIES OF SIMPLE INSPECTION. 323 



to establish, without any appeal to experience, the most general 

 truths of experimental physics. 



Take, for example, two of the most elementary of all laws, 

 the law of inertia and the first law of motion. A body at rest 

 cannot, it is affirmed, begin to move unless acted upon by 

 some external force : because, if it did, it must either move up 

 or down, forward or backward, and so forth ; but if no out- 

 ward force acts upon it, there can be no reason for its moving 

 up rather than down, or down rather than up, &c., ergo, it will 

 not move at all. 



This reasoning I conceive to be entirely fallacious, as indeed 

 Dr. Brown, in his treatise on Cause and Effect, has shown 

 with great acuteness and justness of thought. We have 

 before remarked, that almost every fallacy may be referred to 

 different genera by different modes of filling up the suppressed 

 steps ; and this particular one may, at our option, be brought 

 under petitio principii. It supposes that nothing can be a 

 " sufficient reason" for a body's moving in one particular 

 direction, except some external force. But this is the very 

 thing to be proved. Why not some internal force ? Why not 

 the law of the thing's own nature ? Since these philosophers 

 think it necessary to prove the law of inertia, they of course 

 do not suppose it to be self-evident ; they must, therefore, be 

 of opinion that, previously to all proof, the supposition of a 

 body's moving by internal impulse is an admissible hypothesis ; 

 but if so, why is not the hypothesis also admissible, that the 

 internal impulse acts naturally in some one particular direc- 

 tion, not in another ? If spontaneous motion might have been 

 the law of matter, why not spontaneous motion towards the 

 sun, towards the earth, or towards the zenith ? Why not, as 

 the ancients supposed, towards a particular place in the uni- 

 verse, appropriated to each particular kind of substance? 

 Surely it is not allowable to say that spontaneity of motion is 

 credible in itself, but not credible if supposed to take place in 

 any determinate direction. 



Indeed, if any one chose to assert that all bodies when 

 uncontrolled set out in a direct line towards the north pole, 

 he might equally prove his point by the principle of the 



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