Fallacies to Avoid
The five principal fallacies: Equivocation, Composition and Division, Evading the Point, Begging the Question, and False Cause — with examples and principles for detecting each.
A fallacy is an inference that appears formally valid but is not. Five principal fallacies are examined. Equivocation uses a term in different senses in different parts of the argument (the middle term shifts meaning, violating Law 1). Composition and Division treats together what should be taken separately (composition) or separately what should be taken together (division). Ignoratio Elenchi (Evading the Point) proves something other than what is at issue — the classic misdirection fallacy of all disputation. Petitio Principii (Begging the Question) assumes in the premises what is to be proved in the conclusion. Non Causa pro Causa (False Cause) treats as a cause of a thing what is not really its cause — including the post hoc ergo propter hoc error ('after this, therefore because of this'). Detection requires identifying which specific law of correct thinking is violated.
When a fact has been proved by clear-cut and justified reasoning — deductive or inductive — it is said to be demonstrated. When one or more premisses are merely probable, the conclusion is probable, not demonstrated. When one or more premisses are certainly false, though bearing the attractive appearance of truth, or when the conclusion is not justified by the premisses though it seems at first sight to be — the argument is a sophistry or fallacy.
The conclusion of a fallacy may express a truth (in which case the conclusion is illogical) or a falsehood (in which case it is erroneous or simply an error).
This Chapter discusses the more commonly used fallacies so that the student may readily recognise unjustified reasoning without needing to make a laborious formal analysis every time.
1. Equivocation
This fallacy employs equivocal or vague terms that cause the argument, when reduced to syllogistic form, to offend against the First Law of Terms: “Three terms there must be, neither more nor less.” It smuggles in a fourth term under cover of a single word used in two senses.
Obvious example:
A box is a wooden case. A blow on the ear is a box. Therefore, a blow on the ear is a wooden case.
This contains four terms. More dangerous is equivocation through vague terms — words like democracy, brotherhood, progress, prosperity, education, materialism, optimism, values, higher life — terms used constantly by people who could not frame an adequate definition of any one of them.
Example:
Prosperity promotes progress. Active commerce is prosperity. Therefore, active commerce promotes progress.
What is meant by progress? What by prosperity? To different people these words carry wildly different shades of meaning — some directly opposed. As it stands, this syllogism is valueless unless its terms are accurately explained.
Two practical principles:
- Never use vague terminology. Use terms in a clear and precise sense, and define them when necessary.
- Never allow an opponent to “get away” with a vague argument. Make him define his terms.
2. Composition and Division
This fallacy attributes a predicate to a qualified subject when that predicate is applicable only to the unqualified subject — or vice versa.
Fallacy of Division: applies to the divided (unqualified) subject a predicate that belongs only to the composed (qualified) subject.
Sinners cannot get to heaven. St. Augustine was a sinner. Therefore, St. Augustine could not get to heaven.
The predicate “those who cannot get to heaven” applies properly to unconverted sinners, not to sinners as such. The fallacy divides the subject from its qualifier unconverted and applies the predicate as though the qualifier were absent. Supply the qualifier and the syllogism yields four terms; no conclusion is possible.
Fallacy of Composition: applies to the composed (qualified) subject a predicate that belongs only to the divided (unqualified) subject.
To make blind men see is to do a contradictory thing. Our Lord made blind men see. Therefore, Our Lord did a contradictory thing.
In the major premiss, blind men must be taken in its composed sense — “men as blind” cannot see; for that would be a contradiction. But in the minor premiss, the term ought to be taken in its divided sense — “those whom the Lord caused to cease to be blind.” The fallacy, however, holds the term composed in both premisses. The syllogism involves equivocation, presents four terms, and permits no conclusion.
Thousands of intriguing arguments in books and lecture-halls involve these fallacies without being obviously false. The student must be vigilant.
3. Evading the Point
This fallacy offers argument or proof for something other than the point at issue. The logician’s Latin name: ignoratio elenchi (ignorance of the refutation).
Example: those who set out to prove that Catholics are idolatrous and spend their entire effort proving that images are not to be adored — a matter on which every Catholic would agree. They have not engaged the actual point.
Another example: when the evolutionist is asked “How did human life come into being from lower life-forms?” and replies: “It took ages and aeons for the process of development to unfold; lower life-forms gradually developed variations, these became fixed, and after millions of years new species emerged.” This amounts to:
Inquirer: “How did you get here?” Traveller: “It took me a long time to arrive.”
The point is not how long the process took, but how the transition from one kind of being to another is possible.
Similarly, those who explain a body by saying it is made up of atoms do not explain body as such; a small body presents the same metaphysical question as a large one.
Two principles for guarding against this fallacy:
- Be perfectly clear about the exact point at issue.
- Hold the adversary strictly to that point. Do not permit him to wander off and prove something else, then claim victory.
4. Begging the Question
This fallacy (petitio principii) assumes as proved the very point at issue, and draws from it arguments to establish its own truth. The assumption is usually covert — otherwise the fallacy would be open error.
In its most evident form, this fallacy is the vicious circle: proving A by B, and B by A. Descartes committed this error: he proved the existence of God by the testimony of our reason, then proved reason valid by the perfection of God (Who could not give us deceiving faculties).
Example from Weismann (German biologist):
“…we must assume natural selection to be the principle of the metamorphoses, because all other apparent principles of explanation fail us, and it is inconceivable that there should be another capable of explaining the adaptation of organisms without assuming the help of a principle of design…”
The question at issue is the existence or non-existence of a designer of the universe. The argument assumes it is inconceivable that there should be a design-principle — which is precisely what is in question — and uses this assumption as a premiss to exclude design.
5. False Cause
This fallacy (post hoc, ergo propter hoc — “after this, therefore because of this”) presents as the cause of a fact what has merely preceded or accompanied it.
Example: the fall of the Roman Empire was attributed by pagans — and by Gibbon — to the rise of Christianity. Christianity preceded the fall and the two were contemporaneous in certain respects; but temporal succession and accompaniment are not sufficient evidence of causation.
Other fallacies whose names are self-explanatory: False Analogy, Defective Induction, Incomplete Enumeration, False Assumption.
Summary of the Chapter
This Chapter has taught us to be on guard against the more common forms of logical error. The lesson has a twofold value: it makes us careful in constructing our own arguments, and it enables us to recognise illogicality in the arguments of others.
The two most common and most powerful enemies of correct thinking are Equivocation (especially through vague terminology) and Evading the Point — the one corrupting the very terms of an argument, the other misdirecting the whole inquiry. Mastery of these two, aided by a firm grasp of what constitutes a correct syllogism, gives the student the practical equipment for evaluating the arguments he encounters daily — in reading, in conversation, and in public life.