FALLACY. The incorrect performance of the process of reasoning, so as to lead to error, is said to be a fallacy. The science of logic reduces sound reasoning to certain rules, and when any of these rules is violated, a logical fallacy is the result. There is always included in logical treatises a chapter on fallacies, in which the several kinds are classified and illustrated. In the old writers, there was always a division into two classes, according as the error lay in the form of the reasoning, or in the matter; the for mal were entitled in dictione, or those appearing in the expression; the material were entitled extra dietionem, implying that the fault could not be detected from the language, but must be sought in a consideration of the meaning. or subject-matter. As some of the designations employed in detailing these various kinds of erroneous reasoning have passed into common use, we shall first give a short notice of the ancient classification.
The formal, or those in dietione, were direct breaches of the laws of syllogism, or of argumentation from premises.
The fallacy of undistributed middle is one of the cases where what is called the middle term of a syllogism is used in two senses. " A term is said to be ' distributed ' when it is taken universally, so as to stand for everything it is capable of being applied to; and, consequently, is 'undistributed' when it stands for a portion only of the things desk* nated by it. Thus, 'all food,' or every kind of food, are expressions which imply the distribution of the term ' food;" some food,' would imply its non-distribution.' In such a proposition as " all food is obtained from the vegetable or animal kingdoms," the term is distributed, because it is meant to be affirmed of every article used as food, that such article is derived from one or other of these two sources. But when we say " food is necessary for life," we mean only a limited number of articles. Hence such a syllogism as the following: "Food is necessary to life; corn is food; therefore, corn is necessary to life," is faulty from undistributed middle; the major proposition, " food is necessary," etc., has the form of a universal proposition, with the reality of a particular one.
The cequivoeatio, or ambiguous middle, is the case where a word is used in two sense.
SO different as to.Ogitve properly no middle term, and, therefore, no dounecting lick between the premises and the conclusion. A favorite example of this is the following: "Every dog runs on four legs; Sirius. (the do•star) is a dog; therefore Sirius runs on four legs." This is merely playing with the ambiguity of a word. Dr. Whately has shown that this fallacy may often arise with words derived from the same root, but acquiring from usage different significations; thus, "projectors are unfit to be trusted; this man has formed a project, therefore he is unfit to be trusted;" where the argument supposes that the meaning of " projector " and " one who has formed a project is the same, which it is not.
The fallacy of composition and dirision arises by using a word distributively that is meant collectively; thus, " five is equal to two and three; two and three are even and odd; therefore five is even and odd." "The fallacy of accent was au ambiguity arising from pronunciation. Thus, by a false accent in reading the commandment, " thou shalt not bear false witness against thy neighbor," it might be suggested that subornation is not forbidden, or that anything false except evidence is permitted, or that false evidence may be given for him, or that it is only against neighbors that false witness is not to be borne." The fallacies accidentis is still a form of the ambiguous middle. It is when we con clude of a thing something that is only true of it accidentally, as, " wine is pernicious. therefore it ought to be forbidden." The premise is true only of the immoderate use; the conclusion refers to its use in every form. Another fallacy, the converse of this, is arguing a ditto secundum quid ad dictum simpliciter (passing from what is true in some respect to what is true absolutely). Of this the stock example is: " What you bought yesterday you eat to-day; you bought raw meat yesterday, therefore you eat raw meat to-day." The most usually quoted of the second class of fallacies--extra the following: Ignoratio elenchi. or "ignorance of the refutation." This means mistaking the point in dispute; or proving something that an opponent does not deny. This is common enough in controversy. See an example in point in ETHICS.