528 FALLACIES. 



§ 5. Proceeding with the enumeration of a priori fallacies, and endeav- 

 oring to arrange them with as much reference as possible to their natural 

 affinities, we come to another, which is also nearly allied to the fallacy pre- 

 ceding the last, standing in the same relation to one variety of it as the 

 fallacy last mentioned does to the other. This, too, represents nature as 

 under incapacities corresponding to those of our intellect ; but instead of 

 only asserting that nature can not do a thing because we can not conceive 

 it done, goes the still greater length of averring that nature does a particu- 

 lar 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 scientific authorities for demonstrating a priori the laws of physical 

 phenomena. A phenomenon must follow a certain law, because we see no 

 reason why it should deviate from that law in one way rather than in an- 

 other. This is called the Principle of the Sufficient Reason ;* and by 

 means of it philosophers often flatter themselves that they are able to es- 

 tablish, without any appeal to experience, the most general truths of ex- 

 perimental physics. 



Take, for example, two of the most elementary of all laws, the law of in- 

 ertia and the first law of motion. A body at rest can not, 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 outward force acts upon it, there can be no reason for its mov- 

 ing up rather than down, or down rather than up, etc., 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 aouteness and 

 justness of thought. We have before remarked, that almost every fallacy 

 may be referred to difEerent genera by different modes of filling up the 

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

 under j^etitio princlpii. It supposes that nothing can be a "sufficient rea- 

 son" 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 iriternal 

 foi-ce? Why not the law of the thing's own nature ? Since these philoso- 

 phers 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 pre- 

 viously 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 ad- 

 missible, that the internal impulse acts naturally in some one particular di- 

 rection, not in another ? If spontaneous motion might have been the law 

 of matter, why not spontaneous motion toward the sun, toward the earth, 

 or toward the zenith ? Why not, as the ancients supposed, toward a par- 

 ticular place in the universe, appropriated to each particular kind of sub- 

 stance? 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 deter- 

 minate direction. 



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

 out in a direct line toward the North Pole, he might equally prove his point 

 by the principle of the Sufficient Reason. By what right is it assumed 

 that a state of rest is the particular state which can not be deviated from 

 without special cause ? Why not a state of motion, and of some particular 

 sort of motion ? Why, may we not say that the natural state of a horse 



* Not that of Leibnitz, but the principle commonly appealed to under that name by mathe- 

 maticians. 



