NATURE AND CHARACTERISTICS OF INFERENCE 401 



We are now in a position to examine explicitly a difficulty 

 which is at least as old as Sextus Empiricus (A.D. 200), but which 

 owes its currency in modern times to John Stuart Mill : the ob 

 jection that every syllogism, if regarded as a means of proof, in 

 volves the fallacy of Petitio Principii. 



195. THE SYLLOGISM AND THE &quot;PETITIO PRINCIPII&quot;. The 

 fallacy known as Petitio Principii, or Begging the Question, or 

 Arguing in a Circle (the &quot; Circulus Vitiosus&quot;\ is a fallacy that is 

 liable to be committed in attempting to prove the truth of a given 

 judgment. It consists in assuming, in one or other of many 

 possible ways, in the premisses of our demonstrative syllogism, 

 and utilizing for our proof, a knowledge either of the conclusion 

 itself which we want to prove, or of some other judgment whose 

 truth will be admitted only through a knowledge of this conclu 

 sion, and as an inference from the latter. The way in which the 

 fallacy is alleged to be committed in the syllogism the only form 

 of the fallacy which concerns us here is the following : Not only 

 is the truth of the conclusion really and objectively involved in the 

 truth of the premisses, in the sense that the conclusion would not 

 be true unless the premisses were true ; this is the case in every 

 formally valid syllogism : if the conclusion were false the pre 

 misses would be false (148) : this, however, does not involve any 

 fallacy whatsoever ; but it is further alleged that in no case can 

 we know the premisses to be true unless we already know that the 

 conclusion is true, i.e. unless the conclusion is subjectively in the 

 premisses ; or, in no case can we establish or prove the premisses 

 unless the conclusion has been already established or proved ; or, 

 in no case can we establish the universal premiss without simul 

 taneously, and from exactly the same source, establishing the con 

 clusion : so that the syllogism never actually proves its conclusion, 

 but is a mere reminder that the latter was already established in 

 the process of establishing the universal premiss. 



We have now to inquire whether these allegations are always 

 and necessarily true of all syllogisms. Whether a given syllogism 

 involves the fallacy in question or not, will evidently depend on the 

 source from which we derive our knowledge of the premisses 

 especially of the universal premiss, which, therefore, is the only 

 one we need consider here. If we cannot obtain this knowledge 

 independently of the conclusion we commit the fallacy ; other 

 wise we do not. 



Mill s contention is that we can never reach a knowledge of 

 VOL. I. 26 



