ERROR AND FALLACIES 317 



credibility of a hostile witness by showing that the latter has a 

 criminal record, and that his evidence is unreliable. Sometimes, 

 too, we may be quite satisfied to show that, whatever about our 

 own position, that of our adversary at all events is unsustainable, 

 being inconsistent with the latter s own principles or admissions. 

 This is refuting him &quot; out of his own mouth,&quot; from his own ad 

 missions in theory or in practice, and without committing ourselves 

 to the truth of such admissions (254, &). Thus, Christ silenced 

 those who blamed Him for curing on the sabbath by asking them 

 which of them, if his ox or his ass had fallen into a ditch, would 

 not pull it out on the sabbath. 



(6} PETITIO PRINCIPH, or &quot; begging the question? is the fallacy 

 of assuming as a premiss, in some form or other, either the very 

 proposition to be proved, or a proposition which can be proved 

 only by means of the latter. It is, therefore, a fallacy incident to 

 demonstration, proof, scientific explanation. The student will be 

 familiar with it already, from what has been said on the Nature 

 of Inference (195-6) and on the Uniformity of Nature (224); but 

 it can be so hidden and harmful that it calls for special analysis. 

 The title of the fallacy recalls the language of the dialectical 

 disputation (205), in which the disputant sought his premisses, for 

 the refutation of his adversary, among the latter s admissions. If 

 he endeavoured to get his adversary to admit the very point in dis 

 pute, or used this as a premiss against the latter, or used some 

 other proposition which he could establish only by means of such 

 an admission, his refutation would be sophistical : he would have 

 &quot; begged the question &quot; ; he would have assumed as a premiss 1 a 

 proposition which he could not legitimately assume for his pur 

 pose. 



Aristotle distinguishes five ways in which the fallacy can 

 occur: (i) by assuming the very proposition itself to be proved, 

 usually under cover of synonyms ; (2) by assuming, for the proof 

 of a particular proposition, a universal principle which cannot be 

 itself established except through a knowledge of that particular ; (3) 



1 The universal propositions accepted as starting-points of disputation were 

 called by the Scholastics principia (cf. WELTON, op. cit., ii., p. 283). These were 

 either self-evident axioms, or demonstrable and universally admitted truths. The 

 fallacy of assuming as a principle something which is not such, is dealt with below 

 as &quot;Undue Assumption of Axioms&quot;. It is not quite the same as &quot;begging the 

 question,&quot; i.e. assuming either the conclusion to be proved, or some proposition 

 which can be proved only by means of this conclusion : this would be better called 

 Petitio Quaesiti, or Petitio Quastionis; but the traditional name, Petitio Principii, is 

 too well established to be disturbed by any alteration of usage. 



