CHAPTER VI. 



FALLACIES OF RATIOCINATION . 



1. WE have now, in our progress through the classes 

 of Fallacies, arrived 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 

 most satisfactorily treated in a work familiar to almost all, in 

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

 tions, 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 premises; 

 the fallacies connected with the conversion and sequipollency 

 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 proposition, 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 clearly enunciated with 

 out being detected. And so with another form of fallacy, not 



