498 FALLACIES. 



CHAPTER VI. 



FALLACIES OF RATIOCINATION. 



§ 1. We have now, in our progress through the classes of 'Fallacies, 

 amved at those to which, in the common books of logic, the appellation 

 is in general exclusively appropriated ; those which have their seat in 

 the ratiocinative or deductive part of the investigation of truth. On 

 these fallacies it is the less necessary for us to insist at any length, as 

 they have been so admirably treated in a work familiar to almost all, 

 in this country at least, who feel any interest in these speculations, 

 Archbishop Whately's Logic. Against the more obvious forms of this 

 class of fallacies, the rules of the syllogism are a complete protection. 

 Not (as we have so often said) that the ratiocination cannot be good 

 unless it be in the form of a syllogism ; but that, by showing it in that 

 form, we are sure to discover if it be bad, or at least if it contain any 

 fallacy of this class. 



§ 2. Among Fallacies of Ratiocination we ought, perhaps, to include 

 the errors committed in processes which have the appearance only, not 

 the reality, of an inference from premisses ; the fallacies connected with 

 the conversion and aequipollency of propositions. I believe errors of 

 this description to be far more frequently committed than is generally 

 supposed, or than their extreme obviousness might seem to admit of. 

 For example, the simple conversion of an universal affirmative propo- 

 sition, All A are B therefore all B are A, I take to be a very common 

 form of error : though committed, like many other fallacies, oftener in 

 the silence of thought than in express words, for it can scarcely be 

 cleai-ly enunciated without being detected. And so with another form 

 of fallacy, not substantially different from the preceding ; the eiToneous 

 conversion of an hypothetical proposition. The proper converse of an 

 hypothetical proposition is this : If the consequent be false, the ante- 

 cedent is false ; but this, If the consequent be true, the antecedent is 

 true, by no means holds good, but is an error corresjjonding to the 

 simple conversion of an universal affirmative. Yet hardly anything is 

 more common than for people, in their private thoughts, to draw this 

 inference. As when the conclusion is accepted, which it so often is, 

 for proof of the premisses. That the premisses cannot be true if the 

 conclusion is false, is the unexceptionable foundation of the legitimate 

 mode of reasoning called a reductio ad ahsurdtim.. But men continu- 

 ally think and express themselves as if they also believed that the 

 premisses cannot be false if the conclusion is true. The truth, or sup- 

 posed truth, of the inferences which follow from a doctrine, often ena- 

 bles it to find acceptance in spite of gross absurdities in it. How many 

 systems of philosophy, which had scarcely any intrinsic recommenda- 

 tion, have been received by thoughtful men because they were sup- 

 posed to lend additional support to religion, morality, some favorite 

 view of politics, or some other cherished persuasion ? not merely be- 

 cause their wishes were thereby enlisted on its side, but because its 

 leading to what they deemed sound conclusions appeared to them a 

 strong presumption in favor of its truth : though the presumption, when 

 viewed in its true light, amounted only to the absence of that particular 



