FALLACIES OF SIMPLE INSPECTION. 365 



v 



goes the still greater length of averring that nature 

 does a particular thing, on the sole ground that we 

 can see no reason why she should not. Absurd as 

 this seems when so plainly stated, it is a received 

 principle among philosophers for demonstrating a 

 priori the laws of physical phenomena. A pheno- 

 menon must follow a certain law, because we see no 

 reason why it should deviate from that law in one 

 way rather than in another. This is called the Prin- 

 ciple of the Sufficient Reason; and by means of it 

 philosophers often flatter themselves that they are 

 able 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 p but if no outward 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. Q. E. D. 



This reasoning I conceive to be entirely falla- 

 cious, 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 ex- 

 ternal 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 



