Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

St. Thomas wrote an opusculum, On Fallacies, which is number 39.¹ Almost everything pertaining to fallacies has been explained while discussing the defects of syllogisms, since fallacies are defects of consequence.

Therefore, we shall only indicate here how fallacies are reduced to those defects. Some fallacies pertain to the word, others to the thing signified. Fallacies on the side of the word are reduced to equivocation, because this fallacy is generated by the appearance of sameness in a word: either the same word signifies many things, or one word resembles another, either in pronunciation or in signification; or, finally, the very composition of a statement is equivocal and amphibolous, as happens in cases interpreted in the composite and divided senses, which is one of the fallacies of diction.

We reduce all fallacies pertaining to diction to equivocation. There are six modes of fallacy, three of which are found in simple terms: equivocation, difference of accent, and figure of diction.

Equivocation occurs, for example, if from the term “dog” I infer that what belongs to the animal also belongs to the constellation. Difference of accent occurs if “to hang,” spoken with a short vowel, and “to weigh,” spoken with a long vowel, are taken as the same; or if “poplar,” with its first vowel lengthened, is taken as the tree, or with that vowel shortened, as a people. Figure of diction is a resemblance of a word’s sound or signification to another word. For example, a word signifying a male may seem to signify a female, as when one says: “A substance colored with whiteness is white; therefore a man, who is a substance colored with whiteness, is white [feminine].” Or a word signifying a quality, for example a difference, may be changed into a “what,” that is, a species; or a word signifying a “such-and-such” essence may be changed into a “this something.” All this pertains to figure of diction.

On the side of discourse, equivocation is also found in three modes. The first is amphibology, which makes the interpretation of a statement ambiguous in composition and division, namely, when the composite sense is changed into the divided, or conversely. When a statement is false in the composite sense, it is called the fallacy of composition; when it is false in the divided sense, it is called the fallacy of division.

Fallacies that do not pertain to diction but to the thing signified are reduced to the other defects listed above. There are seven fallacies of this kind: the fallacy of accident, the fallacy from a qualified sense to an unqualified one, the fallacy of ignorance of the refutation, the fallacy of begging the question, the fallacy of the consequent, the fallacy of non-cause as cause, and the fallacy of several questions as one.

The fallacy of accident is almost the same as a change of appellation. It arises because something is signified as belonging to two things that are one only accidentally; thus accidental unity together with diversity in the thing is the cause of this fallacy. This whole matter is a diversity of appellation, or is reduced to it, whether through a different interpretation of the middle term or otherwise. For example: “I know someone coming; Peter is someone coming; therefore I know Peter.”

The fallacy from a qualified sense to an unqualified one is reduced to the fallacy from what is undistributed to what is distributed, or from what is not amplified to what is amplified without preserving the constant term alongside the amplification. For example: “An Ethiopian is white with respect to his teeth; therefore he is simply white.”

The fallacy of ignorance of the refutation is reduced to a defect of opposition, because things appear to be opposed but are not, since the laws of opposition are not preserved. For example: “The house is closed at night and is not closed during the day; therefore it is closed and not closed.”

The fallacy of begging the question occurs when one supposes or assumes what ought to be proved. For example, if you want to prove that Socrates is Plato’s father, you take as the middle term that Plato is Socrates’ son.

The fallacy of the consequent arises from a defect in the rule: “Whatever follows from the consequent of a good consequence follows from its antecedent.” For we infer from an antecedent that something follows from it because we think it is a consequent and is connected with something else, even though in reality it has no such connection. Therefore, the fallacy of the consequent has a close affinity with the fallacy of accident. For example: “If someone is a thief, he wanders about at night; but you wander about at night; therefore you are a thief.” Or from the opposite of the consequent: “Everything generated has a principle; but the soul is not generated; therefore it has no principle.”

The fallacy of non-cause as cause occurs when something is taken as a middle term that is in reality neither a middle term nor a cause uniting the predicate with the subject, although it seems to be a cause. For example: “Death is corruption; therefore life is generation; therefore living is being generated, because death and life are contraries.” This is not a good cause, however, because they are not contraries but privations.

The fallacy of several questions as one is a deception arising when several things that require different answers are asked in a single question. A single answer therefore cannot satisfy it; rather, the question must be distinguished. For example: “Is an Ethiopian a white man?” “Are honey and gall sweet?” “Are man and horse a rational animal?” If you answer affirmatively with one answer, an inference is drawn against you: “An Ethiopian is a white man; therefore he is white.” If you answer negatively, the inference is: “Therefore he is not a man.” The question must therefore be distinguished: he is a man, but not white.

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