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the Issue; with regard to the conclusion, that of Irrelevant Conclusion, and in either case, Ignoratio Elenchi.
RULES OF INFERENCE
$ 127. All inference, as we have observed, may be resolved into the process of substituting for terms other terms of equivalent ratiocinative value. There is an apparent exception in the case of conversions of propositions, but the exception is only apparent ($ 79). To conform to usage, however, the rule for conversion will be given, though in fact, as explained, the illicit conversion of a proposition is simply a case of illicit substitution of terms.
Rule V. CONVERSIONS TO BE ILLATIVE.
A conversion, to be legitimate, must be illative, i. l., the
truth of the converted must be implied in the original proposition.
The violation of this rule may be called the Fallacy of Conversion, or simply Illicit Conver. sion. It can occur only in the simple conversion of a universal affirmative or a particular negative proposition (e. g., “ Y is X," " Some Y is not X'). In the former case the fallacy will consist in the substitution of genus for species (X for Y) in the subject, and of species for genus (Y for X) in the predicate of a universal affirmative proposition, thus doubly violating the first rule of substitution. In the lat. ter (“ Some Y is not X”) X is substituted for Y in the subject, and Y for X in the predicate, though neither is necessarily, and one at least cannot be, a species of the other; which is a violation of the next rule.
Rule VI. EQUIVALENCE OF TERMS TO BE OBSERVED.
In all substitutions the substituted term must be equivalent in signification-i.l., equivalent in ratiocinative value-to the term for which it is substituted.
The violation of this rule by the substitution of a new term is called the Fallacy of Illicit Substitution.
The rule will cover all cases of legitimate substitution of terms whatever ; but it is obvious, where an ambiguous term is used in a different sense from that originally adopted, that a new term is in fact illicitly substituted. We must add, therefore, as a corollary the following:
Rule VII. THE SENSE OF TERMS TO REMAIN UNALTERED.
Every verbal expression, whether a term or proposition, shall, throughout the ratiocination, be used in the sense originally given to it.
The violation of this rule constitutes what is called the Fallacy of Equivocation, which is to
be regarded as a species of Illicit Substitution; and of this there are two kinds: the first consisting in shifting the sense of an ambiguous term, which is called the Fallacy of Ambiguity; the second, in shifting the meaning of what is called an amphibolous sentence, which is a sentence equivocal by reason of its grammatical construction, as, e. g., the sentence,“ The Duke yet lives that Henry shall depose”; which may mean either that the Duke shall depose Henry, or Henry the Duke. If construed in the former sense, the subject of the proposition is, “ The Duke that shall depose Henry”; for which under the latter construction is substituted, “The Duke that shall be deposed by Henry." This is called the Fallacy of Amphibology, or, perhaps better, of Amphiboly. But these fallacies are of essentially the same nature, and will be classed together under the one head of Equivocation.
THE DOCTRINE OF FALLACIES
DEFINITION AND CLASSIFICATION OF FAL
$ 128. DEFINITION OF FALLACIES.-A fallacy may be defined as a false semblance of valid ratiocination; to which it bears the same relation as hypocrisy, conscious or unconscious, to virtue. Fallacy is therefore a species of error, whose specific difference consists in its semblance of right reasoning and its consequent liability to be mistaken for it.' It may
Hobbes, with his usual acuteness, thus clearly explains the distinction between error and fallacy :
“When we reason with words of general signification (universalibus) and fall upon a general conclusion (conclusionem universalum) which is false, though it be commonly called error, it is indeed an absurdity or senseless speech (oratio insignificans).”- Lev., chap. v. According to this view, all fallacies are absurdities, i. e., they necessarily involve either a contradiction, or the use of non-significant or senseless words.