378 



FALLACIES. 



substantially different from the preceding : the erroneous con- 

 version of an hypothetical proposition. The proper converse 

 of an hypothetical proposition is this : If the consequent be 

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

 true, the antecedent is true, by no means holds good, but 

 is an error corresponding to the simple conversion of an 

 universal affirmative. Yet hardly anything is more common 

 than for people, in their private thoughts, to draw this infe- 

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

 is, for proof of the premises. That the premises cannot be 

 true if the conclusion is false, is the unexceptionable founda- 

 tion of the legitimate mode of reasoning called a reductio ad 

 absurdum. But people continually think and express them- 

 selves, as if they also believed that the premises cannot be 

 false if the conclusion is true. The truth, or supposed truth, 

 of the inferences which follow from a doctrine, often enables 

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

 many philosophical systems which had scarcely any intrinsic 

 recommendation, have been received by thoughtful men be- 

 cause they were supposed to lend additional support to reli- 

 gion, morality, some favourite view of politics, or some other 

 cherished persuasion: not merely because 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 favour of its truth : though the presumption, 

 when viewed in its true light, amounted only to the absence 

 of that particular evidence of falsehood, which would have 

 resulted from its leading by correct inference to something 

 already known to be false. 



Again, the very frequent error in conduct, of mistaking 

 reverse of wrong for right, is the practical form of a logical 

 error with respect to the Opposition of Propositions. It is 

 committed for want of the habit of distinguishing the contrary 

 of a proposition from the contradictory of it, and of attending 

 to the logical canon, that contrary propositions, though they 

 cannot both be true, may both be false. If the error were to 

 express itself in words, it would run distinctly counter to this 

 canon. It generally, however, does not so express itself, and 



