Logical Fallacies



Ambiguity Fallacies

-Equivocation: The same term is used with different meanings.

Example: Jesus commanded his disciples to love. The outcry of the Berkeley demonstrators in the 1960s was "Make love, not war." Therefore, the Berkeley demonstrators were disciples of Jesus.

Explanation: The word "love," as Jesus used it, did not bear the erotic overtones which the Berkeley demonstrators attached to the word in their defiance of established ethical norms, as well as civil authority.  Accordingly, the conclusion does not logically follow from the premises.

 

-Amphiboly (sometimes Amphibology): A grammatical construction that is capable of being interpreted with two different meanings.

Example: Divorce invariably follows marriage. Therefore, marriage is the cause of divorce.

Explanation: The premise can be interpreted in two different senses: (1) that all marriages result in divorce; or (2) that divorce, whenever it occurs, follows marriage. There is a big difference between the two meanings; hence, the premise is ambiguous and confusing to the reader. If the second meaning is intended, qualifying words, such as "whenever it occurs," will remove the ambiguity. If that is the intended meaning, however, then a causal fallacy needs to be addressed.


Causal Fallacies

-Post Hoc (ergo prompter hoc), literally, "After this; therefore because of this": Because one thing follows another, it is frequently assumed (erroneously) that what precedes causes what follows.

Example: Divorce, whenever it occurs, invariably follows marriage. Therefore, marriage causes divorce.

Explanation: Because divorce, whenever it occurs follows marriage, it is assumed that marriage is the cause of divorce. Divorce represents the dissolution of marriage. Marriage, as a divine institution so foundational to human society, can hardly be considered the cause of its own dissolution. This is just as much a fallacy as assuming that birth is the cause of death because the latter is always preceded by the former, or that traffic laws are the cause of speeding because speeders are always arrested after traffic laws have been instituted.


Categorical Fallacies

-Composition: Because the a particular part has a certain attribute or property, it is assumed that the same is true of the whole.

Example: Since many psychologists hold to the materialist theory of the mind, psychology by very definition is the study of the material world, i.e., it embraces all the physical sciences.

Explanation: While it is true that many psychologists hold to a materialist theory of the mind, it does not necessarily follow that the study of the material world, including all the physical sciences, is the direct concern of psychology as an academic discipline. To suggest that psychology embraces all the physical sciences would be a bit of an absurdity, though it may relate to all of them to some degree. What is true of one aspect of the material world, particularly the physical brain, namely that it directly relates to psychology, should not be assumed to be true of all aspects of the material world.


Changing the Subject

-Ad Hominem: An attack upon the character of the man making the argument.

Example: The man's opinion cannot be trusted as he is a former member of the John Birch Society.

Explanation: Because of a common opinion, rightly or wrongly, that the John Birch Society is an extremist, right-wing organization, the association of the man's argument with that organization tends to discredit the man's argument in the public mind. During the era of McCarthianism in American politics, many persons' ideas and reputations were discredited by their association with so-called Communist front groups. While this means of discrediting arguments is fallacious, it is nevertheless common in the political world.

 

Definition Fallacies

-Circular Definition: The word being defined is part of the definition.

Example: A liberal is one who has liberal views on everything.

Explanation: The problem in the above example is that the definition is useless because the word being defined, in this case "liberal," is simply repeated in the definition; hence, the word still requires definition, not simply repetition. The question is: "What is the meaning of 'liberal'?" That question still needs to be addressed.


Distraction Fallacies

-Slippery Slope: Undesirable consequences are associated with the proposition. Sometimes called the "domino theory."

Example: If we make laws against "partial birth abortion," it will only be a matter of time before all laws protecting the life and health of the mother will be rescinded.

Explanation: The slippery slope argument is an appeal to the motive of fear of consequences, in this case of women losing their legal protection during childbirth. As this argument applies to the legislative environment in the U.S., it would seem to be a fallacy inasmuch as pro-life groups typically affirm their legal regard for both mother and child. Since women are fully franchised voters, and many of them directly involved in the legislative and judicial process, it is hardly conceivable that legislation removing their legal protection of life and health could ever be enacted. The concern is to establish the legal rights of the unborn whose rights were removed under the Roe versus Wade Supreme Court decision, and who are unable to defend themselves.

 

Inductive Fallacies

-Hasty Generalization: The sample is too small to support the inductive conclusion

Example: Our class is unanimous in its opinion that the UVA football team is the best in the nation. Therefore, it must be a commonly accepted fact.

Explanation: The class sampling in central Virginia is not a sufficient indicator of national opinion concerning the national collegiate sport. This is obvious from the fact that a number of football teams in other states have superior win-loss records. The opinions of students from other states would have to be considered in order to reach a reliable conclusion. To conclude, on the basis of one college class's opinion in central Virginia, that it is a commonly accepted fact that UVA's football team is the best in the nation represents a hasty generalization.


Missing the Point

-Begging the Question: Simply repeating a previous argument in different form as though it were an additional argument of support for a stated position.

Example: "I know that the man's opinion is wrong because he is obviously incorrect in his thinking!"

Explanation: To say that a man is "obviously incorrect in his thinking" is no different than saying that his "opinion is wrong." To argue in that fashion is simply to spin one's wheels. It fails to advance or improve upon the argument-it is simply begging the question --a kind of special pleading without adequate reasoning. It is like the evolutionist crying out in argumentative despair after his debate opponent has thoroughly discredited his purported evidence, "But I know there are fossil links!"



-Straw Man: The debater sets up an artificial argument that he knows he can easily refute but that simply does not represent the strongest argument of his opponent.

Example: Determinism cannot be true because it is obvious that we all make choices.

Explanation: Some of the strongest determinists are theists, and particularly Christians, who would be the first to acknowledge that we all make choices, though they would insist that the Supreme Being, by very definition, must, in an ultimate sense, determine all things in accordance with His sovereign and gracious will. While we may not appreciate the fact that this truth transcends human understanding, we should not be surprised by it, and it certainly exposes the "strawy" nature of an argument such as the one above. The above argument is superficial, it simply sets up a straw man and knocks it down, in a manner of speaking.

 

Non Sequitor (literally, "It does not follow.")

-Affirming the Consequent: Any argument of the form: If A, then B, B, therefore A.

Example: All swans are white. Donald Duck is white. Therefore, Donald Duck is a swan.

Explanation: The major premise: "All swans are white" is simply an abbreviated way of stating, "If it is a swan, then it is white." The antecedent (A), or the clause that "goes before," is "If it is a swan." The consequent (B), or that which follows, is "then it is white." If the minor premise affirms the consequent (B) as in the proposition: "Donald Duck is white," it results in the invalid conclusion: "Therefore, Donald Duck is a swan." The conclusion which follows from the premises is invalid because the minor premise is fallacious. Any argument of the form: If A, then B, B, therefore A, will produce an invalid conclusion, i.e., one that is not justified from the premises.  Note: To affirm the antecedent changing the minor premise to "Donald Duck is a swan" would result in a conclusion that "Donald Duck is white." In that case the argument would be valid (the conclusion following from the premises), however unsound, since the minor premise "Donald Duck is a swan" is untrue, and so is the major premise that "all swans are white.".

 

-Denying the Antecedent: Any argument of the form: If A, then B, not A, therefore not B.

Example: All swans are white. Donald Duck is not a swan. Therefore, Donald Duck is not white.

Explanation: The major premise: "All swans are white" is simply an abbreviated way of stating, "If it is a swan, then it is white." The antecedent (A), or the clause that "goes before," is "If it is a swan." The consequent (B), or that which follows, is "then it is white." If the minor premise denies the antecedent (A) as in the proposition: "Donald Duck is not a swan," it results in the invalid conclusion: "Therefore, Donald Duck is a not white." The conclusion which follows from the premises is invalid because the minor premise is fallacious. Any argument of the form: If A, then B, not A, therefore not B, will produce an invalid conclusion, i.e., one that cannot be justified from the premises. Note: To affirm the antecedent changing the minor premise to "Donald Duck is a swan" would result in a conclusion that "Donald Duck is white." In that case the argument would be valid (the conclusion following from the premises), however unsound, since the minor premise "Donald Duck is a swan" is untrue, and so is the major premise that "all swans are white."