FALLACIES OF CONFUSION. 459 



more particles ever gravitate more strongly, i. e., are 

 heavier :' ' but, (it may be urged), those which are 

 heaviest are not always more bulky;' 'no, but they 

 contain more particles, though more closely condensed :' 

 ' how do you know that ?' ' because they are heavier:' 

 4 how does that prove it ?' ' because all particles of mat- 

 ter gravitating equally, that mass which is specifically 

 the heavier must needs have the more of them in the 

 same space." It appears to me that the fallacious 

 reasoner, in his private thoughts, would not be likely 

 to proceed beyond the first step *. He would acqui- 

 esce in the sufficiency of the reason first given, " bodies 

 which contain more particles are heavier." It is when 

 he finds this questioned, and is called upon to prove 

 it, without knowing how, that he tries to establish 

 his premiss by supposing proved what he is at- 

 tempting to prove by it. The most effectual way, in 

 fact, of exposing a Petitio Principii, when circum- 

 stances allow of it, is by challenging the reasoner to 

 prove his premisses ; which if he attempts to do, he 

 is necessarily driven into arguing in a circle. 



It is not uncommon, however, for thinkers, and 

 those not of the lowest description, to be led, even in 

 their own thoughts, not indeed into formally proving 

 each of two propositions from the other, but into 

 admitting propositions which can only be so proved. 

 In the preceding example the two together form a com- 

 plete and consistent, though hypothetical, explanation 

 of the facts concerned. And the tendency to mistake 



* I have found, however, an argument of this exact type in a 

 Bridgewater Treatise: " Ice and silver, under the same volume, 

 contain very unequal portions of matter, the silver being ten times 

 as heavy as the ice. The vacuities in the ice, therefore, must be 

 very much greater than those in the silver." 



