FALLACIES OF RATIOCIXATION. 499 



kind of evidence of falsehood, which would have resulted from its lead- 

 ing by correct inference to somethintr already recoL-nized as false. 



Again, the very frequent error in conduct, of mistaking reverse of 

 wrong for right, is the practical form of a logical error with respec! to 

 the Opposition of Propositions. It is committed for want of the habit 

 of distinguishing the contrar;/ of a proposition from the contradictory 

 of it, and of attending to the logical canon, that contrary propositions, 

 though they camiot both be true, may both be false. If the eiTor 

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

 canon. It generally, however, does not so c-xpress itself, and to com- 

 pel it to do so is the most effectual method of detecting and exposing it. 



§ 3. Among Fallacies of Ratiocination are to be ranked, in the first 

 place, all the cases of vicious syllogism laid down in the books. These 

 generally resolve themselves into having m»re than three terms to the 

 syllogism, either avowedly, or in the covert mode of an undistributed 

 middle term or an illicit process of one of the two extrc>mes. It is 

 not, indeed, very easy fully to convict an argiftnent of falling under 

 any one of these vicious cases in particular ; for the reason already 

 quoted from Archbishop Whately, that the prcmiss(!S are seldom for- 

 mally set out: if they were, the fallacy would impose upon nobody; 

 and while they are not, it is almost always to a certain degree oi)tional 

 in what manner the suppressed link shall 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 

 syllogism good by introducing a false premiss ; 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 premisses he is prepared to maintain. The eletlion 

 made, there is generally so little difficulty in seeing whether the con- 

 clusion follows from tl>e premisses set out, that we might without much 

 logical impropriety have merged this fourth class of fallacies in the 

 fifth, or Fallacies of Confusion. 



4 



§ 4. Perhaps, however, the commonest, 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 chani^ini^ the premisses. A proposi- 

 tion is proved, or an acknowledged truth laid down, in the first part of 

 an argumentation, and in the second a further argument is founded not 

 upon the same proposition, but upon some other, resembling it suffi- 

 ciently to be mistaken for it. Instances of this fallacy will be found 

 in almost all the argumentative discourses of unprecise thinkers ; and 

 we need only here advert to one of the obscurer forms of it, recognized 

 by the schoolmen as the fallacy h dicta secundum quid ad dictum sim- 

 pliciter. This is committed wlien, in the premisses, a proj)osition is 

 asserted with a qualification, and the qualification lost sight of in the 

 conclusion ; or oftener, when a limitation or condition, though not as- 

 serted, is necessary to the truth of tin; proposition, but is forgotten 

 when that proposition comes to be employed a.s a premiss. Many of 

 the bad arguments in vogue belong to this class of error. The premiss 



