68 BETIEWS — ANALYTICAL STATICS, 



"All bodies are capable of motion (sont mobiles), but matter 

 cannot spontaneously move itself, for there is no reason why a parti- 

 cle should begin to move in one direction rather than another. It is 

 in fact a matter of ordinary experience that when a body is passing 

 irom a state of rest to a state of motion, we can always attribute 

 the change to the action of some external cause." 



This ' external cause' is further explained by Poisson, as one 

 "sans laquelle nous concevons que ce corps pourrait d'ailleurs 

 exister." 



Xow the sentence above quoted really appeals to two utterly dif- 

 ferent sources for support of the main proposition. The first argu- 

 ment is what we should say might be called an argumentum ad igno- 

 rantiam. "We should object to it, not only because it is using a very 

 dangerous argument on very doubtful ground, but because it fairly 

 brings us into collision with the metaphysician. We say that it is 

 a very dangerous argument ; and we say this because we coneeive 

 that an appeal is really made here to the reader's own mind to form 

 an idea a priori of what necessarily must be the nature of material 

 bodies — an appeal, which in many cases would obviously lead to a 

 wrong result : which is in fact virtually an abandonment of the in- 

 ductive method. If any one from long familiarity with the reason- 

 ing here employed should be inclined to defend it, we would refer 

 him, as an easy reductio ad absurdum ; to the use made of this 

 mode of arguing by Mr. Gregory, who employs it to shew that the 

 'atom' of chemistry is most probably spherical, " since no reason can 

 be assigned why one dimension should exceed another." It is indeed 

 very difficult to set any formal limitation to the cases in which this 

 argument may be safely used. Certainly, however, it would be a 

 very unsafe guide in speculating upon the physieal properties of mat- 

 ter, in which manner it is really used here. The second objection 

 to the argument is perhaps even more formidable. At any eost we 

 must keep clear of Metaphysics in the commencement of a physical 

 science. If the fimdamental truth of Statics is to be mads to 

 rest upon popular conceptions of time or space, any writer on Meta- 

 physics who attacks those conceptions involves our system of Statics 

 also in doubt. This should not be , if for example a Metaphysician 

 insists that space and time instead of being real existences are 

 merely modes of thought necessary to a finite mind, we should be 

 able to answer (whatever may be our opinion of his theory) that 

 our science is occupied exclusively with results of which these samer 



