FALLACIES OF RATIOCINATION. 



527 



thing 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 premises. That the pre- 

 mises cannot be true if the conclusion 

 is false, is the unexceptionable foun- 

 dation of the legitimate mode of rea- 

 soning called reductio ad absurdum. 

 But people continually think and 

 express themselves as if they also 

 believed that the premises cannot be 

 false if the conclusion is trne. The 

 truth, or supposed truth, of the in- 

 ferences 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 

 because they were supposed to lend 

 additional support to religion, mora- 

 lity, some favourite view of politics, 

 or some other cherished persuasion ; 

 not merely because their wishes were 

 thereby enlisted on its side, but be- 

 cause 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 fron» its leading by cor- 

 rect 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 Oppo- 

 sition of Propositions. It is com- 

 mitted for want of the habit of dis- 

 tinguishing the contrary of a proposi- 

 tion 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 to compel it to do so is the most 

 effectual method of detecting and ex- 

 posing it. 



§ 3. Among Fallacies of Ratiocina- 

 tion are to be ranked in the first place 

 all the cases of vicious syllogism laid 

 down in the books. Tliese generally re- 

 solve themselves into having more than 

 three terms to the syllogism, either 

 avowedly, or in the covert mode of an 

 undistributed middle term, or an illi- 

 cit process of one of the two extremes. 

 It is not, indeed, very easy fully to 

 convict an argument of falling under 

 any one of these vicious cases in par- 

 ticular ; for the reason already more 

 than once referred to, that the pre- 

 mises are seldom formally set out : 

 if they were, the fallacy would impose 

 upon nobody ; and while they are not 

 it is almost always to a certain degree 

 optional in what manner the sup- 

 pressed link should be filled up. The 

 rules of the syllogism are rules for 

 compelling a person to be aware of 

 the whole of what he must undertake 

 to defend if he persists in maintain- 

 ing his conclusion. He has it almost 

 always in his power to make his syllo- 

 gism good by introducing a false pre- 

 mise ; and hence it is scarcely ever 

 possible decidedly to affirm that any 

 argument involves a bad syllogism : 

 but this detracts nothing from the 

 value of the syllogistic rules, since it 

 is by them that a reasoner is compelled 

 distinctly to make his election what 

 premises he is pi-epared to maintain. 

 The election made, there is generally 

 so little difficulty in seeing whether 

 the conclusion follows from the pre- 

 mises set out, that we might without 

 much logical impropriety have merged 

 this fourth class of fallacies in the 

 fifth, or Fallacies of Confusion. 



§ 4. Perhaps, however, the com- 

 monest and certainly the most dan- 

 gerous fallacies of this class, are those 

 which do not lie in a single syllogism, 

 but slip in between one syllogism and 

 another in a chain of argument, and 

 are committed by changing the pre- 

 mises. A proposition is proved, or an 

 acknowledged truth laid down, in the 

 first part of an argumentation, and 

 in the second a further argument i? 



