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A logical fallacy is an error in logical reasoning. While the common usage of the word fallacy would include any error in reasoning, in logic a fallacy is defined at a particularly deceptive argument which seems correct, but upon further examination is found to be incorrect.[1]

Logical fallacies fall into two general categories: formal fallacies and informal fallacies. Formal fallacies apply to deductive arguments, and are those which relate to an improper application of a rule, whereas informal fallacies apply to inductive arguments, and are those which involve the improper use of the content of an argument.

Logical fallacy is not the same as lying. A lie in logic is a premise that one offers while knowing that it is false.

The following is a catalog of informal logical fallacies, with each one given a separate heading, to make them easier to cite in other articles.

Catalog of Logical Fallacies

Argumentum ad hominem

Main Article: Ad hominem

(Latin: "Argument directed toward the man"); An argument that attacks the character of one holding a contrary view, rather than attacking the view itself. Name-calling is a particularly crude form of this fallacy. Argumentum ad hominem is usually an attempt to accuse the other person of lying without saying so directly, or saying that his premises are somehow incompetent, irrelevant, or immaterial, or simply unreliable because they are self-serving. However, this is not good logic. If one suspects another of lying, then one should attempt to disprove the premises. One can raise a suspicion of deliberate falsehood on the part of another who has offered deliberate falsehood in the past. But the most that that can accomplish is to suggest that the other person's premises are unreliable and therefore require third-party corroboration.

Overgeneralization

An argument that attempts to apply a principle to circumstances beyond the scope of its original formulation or of any reliable demonstration. The application of Isaac Newton's original theory of relative velocity (which is that it was simply additive) to speeds approaching that of light was an overgeneralization, as relativity would later show.

Overgeneralization also is the central problem in trying to conclude something about a population from a non-representative sample, or about a larger group from a non-representative subset of that group.

Non sequitur

Main Article: Non sequitur

(Latin: "It does not follow"); An argument which moves from a premise to a conclusion without showing a valid connection.

Proof by authority

Also known as argumentum ab auctoritate (Latin "Argument proceeding from clout"), this is an argument that a person bases on authority, either his own or that of another person, rather than on the merits of the position. When the authority involved is a relevant source who has access to more information about the topic than the people discussing it, then the argument becomes a citation. However, a valid citation must be in an area of study, research, or mental discipline in which the authority being cited is a recognized expert.

A classic example of argument from authority is a reference to a celebrity or religious leader for their opinion on a matter of science or public policy, when that celebrity or cleric has never adequately studied the subject. This kind of argument also appears in commercial advertising or political electioneering.

Proof by numbers

This is an argument that a person bases on the numbers of people holding to its conclusion, rather than on the premises that might support that conclusion. "Ninety nine point nine percent of all respondents can't be wrong" is the classic phraseology. One does not effectively disprove such an argument by challenging the numbers. Instead, one reminds the other person that the numbers of people holding to any given conclusion are irrelevant to establishing the truth or falsehood of that conclusion. History is in fact replete with multiple examples, too many to mention here, of conclusions that memorable scientists and other great discoverers have shown to be false even though large numbers of people believed them. Antoine Lavoisier, who disproved "phlogiston" as the principle of fire, was one such person. Christopher Columbus and Ferdinand Magellan make two more.

Proof by assertion

An argument that states something as true without offering supporting evidence or attempting to construct a valid argument. The typical form of this "argument" is the "conclusion" of a non sequitur.

Circular reasoning

Circular reasoning is a form of proof-by-assertion in which one asserts a premise, then asserts a conclusion from that premise (directly or indirectly), and then tries to show that the last conclusion supports the original premise.

Argument from silence

Argument from silence (in Latin, argumentum ab silencio) is an assertion that a proposition is false merely because no one has yet asserted evidence to confirm it--or that a proposition is true merely because no one has yet asserted evidence to contradict it. "The absence of evidence does not constitute evidence of absence." The charge of argument from silence applies in a situation in which no reasonable attempt has yet been made to develop evidence to confirm or contradict the proposition in question.

This is the opposite extreme from "manufacturing facts to support a theory," described below. The best aid to striking a balance that avoids these two opposing fallacies is Occam's razor.

Proving too much

A form of overgeneralization in which one attempts to use a set of premises to sustain more conclusions than he can reasonably sustain using the argument presented.

Double standard

The unequal use of a criterion, and its different application in different cases. This is actually a form of hypocrisy.

Your theory does not work under my theory, so your theory must be wrong

Main article: Your theory does not work under my theory, so your theory must be wrong

The judgment of a theory by the premises and assumptions of another theory, rather than against its own premises and assumptions. This is a variation on proof-by-assertion.

Manufacturing facts from a theory

Main article: Manufacturing facts from a theory

The assertion as fact of an undemonstrated, unobserved idea for no better reason than that a given theory requires that fact. We deal here with the assumption of a fact not in evidence. In the early stages of formulating a model, this sort of behavior is acceptable. But when repeated efforts to demonstrate such a new fact have failed, the proper logical response is to discard or revise the theory, and not merely to assume that the "fact" still exists and "someone hasn't tried hard enough" to find it. This is especially true when someone develops evidence that the inferred "fact" could not possibly exist.

This is yet another form of proof-by-assertion, and is the source of many controversies in modern astronomy, some of them strikingly bitter.

Genetic fallacy

In general, this is the attempt to aver (that is, assert as true) or reject a theory by citing its origins as either reputable or disreputable. The usual expression of this fallacy is "Consider the source!" Thus it becomes a form either of argumentum ab auctoritate or of argumentum ad hominem, depending on whether one seeks to verify or disprove the theory by this method.

When the origin of evidence or of premises is relevant to the reliability of the same, then asking a hearer to "consider the source" is valid. Judges in courts of law, for example, routinely reject as unreliable the testimony of any witness who has demonstrably lied about a point that matters in the case at hand. The facts that such a witness is asserting might still be true, but they cannot stand without corroboration from another, more reliable witness.

But when corroboration is established, the origins of a conclusion, however tainted, become irrelevant.

As an example, Gregor Mendel established the genetic theory that remains current today, even though Mendel's experimental technique was badly flawed, and he even stands accused of falsifying key data. But succeeding scientists, using accepted methods of verification and statistical assessment, have achieved results consistent with this theory. Thus the theory remains valid even though Mendel's original presentation was fraudulent. Any attempt today to discredit Mendel's theory on account of Mendel's sloppy methods would be an example of a genetic fallacy.[2]

Tautology

A tautology (Greek ταυτα tauta, "these") is an argument that becomes a repetition of a definition. Literally it means "the study of this" or "the study of these." Such an argument, or statement, can prove nothing beyond itself and is useless as a premise.

Contradiction

A contradiction is a statement that contradicts its own terms. Aristotle famously stated that contradictions cannot exist. In any case of a contradition, some of the premises must be false.

Contradictions normally form part of a proof by exclusion. But contradictory arguments are some of the most common flawed arguments, and are closely akin to double standards (see above).

Conflation

Conflation is the treatment of two different concepts as one. The most common form of conflation informs the proverbial accusation, "You are comparing apples to oranges."

Loaded question

Main Article: Loaded question

A loaded question is a question that assumes facts, usually unflattering, that are not in evidence, with the intent of trapping the other person into admitting those facts. The classic loaded-question example is "When did you stop beating your wife?" Another example is, "Do you disbelieve in global warming, which 99.9 percent of all reputable scientists now accept?" (It is also an example of proof by numbers of adherents.)

Special pleading

Main Article: Special pleading

Special pleading means applying to other people a set of standards that one is not willing to apply to oneself, without offering sufficient grounds, called the relevant difference, to support such exemption. Special pleading is a special case of the double standard, and is one of the most despicable forms of hypocrisy. A political or military leader who urges his subjects (or those under his command) to observe "iron rations" without similarly depriving himself leaves himself open to a charge of special pleading.

Related examples of improper reasoning

The following categories of poor reasoning are difficult to classify; some might not wish to classify them as logical fallacies per se, but they are still, in most cases, indefensible:

Infinite regression

Main Article: Infinite regression

An infinite regression results when one asserts that a given event caused another, and yet that first event requires another, identical event, to cause it. Panspermia, an alternative to abiogenesis as a proposition about the origin of life, suffers from the infinite-regression flaw so long as it fails to state positively what conditions could have brought about an origin of life on a planet other than the earth, other than the alleged mechanism of the "seeding" of life on the earth itself.

Is infinite regression ever defensible? If so, when? A debate on this issue is now underway.

Logical fallacy and the educational establishment

Today, logic is offered as an elective in college, not as a requirement. Various graduate-school admissions councils (among them the Graduate Management Admissions Council and the Law School Admissions Council) have long recognized this fact and today test specifically for an appplicant's ability to think critically and apply logic.

Thus the major admissions tests (Graduate Management Admission Test and Law School Admissions Test) feature questions on logical reasoning. A typical question would present an argument and ask either:

  1. What the argument assumes,
  2. What sort of evidence, if established, would significantly strengthen (or weaken) the argument, or
  3. Why the argument, as presented, is flawed.

Still other questions present arguments and then ask the test-taker what reasonable conclusions one might draw from them. Occasionally they present sets of two facts that might seem to contradict one another and ask the test-taker to explain how both facts can be true at once.

Many of the arguments presented (the stimuli) and the wrong answer choices offered illustrate classic logical fallacies, often strikingly. Examples of argumentum ad hominem and the closely related fallacy of non sequitur, to name two key errors, abound in these questions.

Yet for all this effort in trying to identify weaknesses in critical thinking, colleges tend to discourage critical thinking in other ways, mainly in that professors often commit the very sort of logical fallacies that these tests, for example, challenge their test-takers with.

Related References

  1. Copi, Irving M. and Carl Cohen. "Introduction to Logic, 12e". Pearson Education: 2005
  2. Genetic Fallacy on The Fallacy Files

See Also

"The Fine Art of Baloney Detection." Sagan, Carl. The Demon-Haunted World, Science As a Candle in the Dark. Ballantine: New York, 1996; pp 201-218.