Logical Fallacies

The following are taken from Stephen Downes and his web page at http://www.datanation.com/fallacies/index.htm.

Fallacies of Distraction

• False dichotomy: Two choices are given when in fact there are more than two.
• False presumption: Because something is not known to be true, presume it to be false.
• Slippery slope: Claim that a small concession is total surrender.
• Complex question: Unrelated points conjoined as a single proposition.

Appeals to Emotions instead of Fact or Logic

• Appeal to fear: Target is persuaded to agree by threats or force. Argumentum ad baculum, or argument based on threat. Argumentum ad metum, appeal to fear.
• Appeal to pity: Target is persuaded to agree by sympathy. Argumentum ad misericordiam.
• Appeal to envy: Target is persuaded to agree by envy. Argumentum ad invidiam.
• Appeal to hatred: Target is persuaded to agree by hatred. Argumentum ad odium.
• Appeal to pride: Target is persuaded to agree by pride. Argumentum ad superbium.
• Appeal to greed: Target is persuaded to focus on the gains and ignore the risks or costs. Argumentum ad edacitam, rapacitam, avaritiam, greed, rapacity, avarice.
• Appeal to ignorance: Target is persuaded to agree if can't prove the contrary. Argumentum ad ignoratiam.
• Consequences: Target is warned of unacceptable consequences.
• Prejudicial language: Value or moral goodness is attached to the author or his position.
• Bandwagon: A proposition is argued to be true because it is widely held to be true. Appeal to the safety of the herd. Argumentum ad populum, or appeal to the mass opinion of the people.
• Authority: Argumentum ad verecundiam. A proposition is argued to be true because it is supported by experts or authorities. This is widely accepted as a method of argument, but strictly speaking, it is a logical fallacy.

Fallacy of Authority

• Recognition: Everyone recognizes the person as an authority, therefore what he says must be true.
• Production: The person has done a great deal of authoritative work, therefore he must be an authority.
• Power: The person is powerful and successful, therefore he speaks with authority, if only by virtue of his position.

Changing the Subject

• Attack the Person (ad hominem):
  • (1) Attack the person's character.
  • (2) Attack the person's circumstances.
  • (3) Argue the person does not practise what he preaches.
• Attack the Authority:
  • (1) Claim the authority is not an expert in the field.
  • (2) Claim experts in the field disagree.
  • (3) Claim the authority was joking, drunk, or in some other way not being serious.
• Anonymous authority: Cite an authority not named
• Style over substance: The manner in which an argument or arguer is presented used as argument to the truth of the conclusion.

Inductive Fallacies

• Hasty generalization: The sample is too small to support an inductive generalization about a population.
• Unrepresentative sample: The sample is unrepresentative of the sample as a whole.
• False analogy: The two objects or events being compared are relevantly dissimilar.
• Fervent denial: The conclusion of a strong inductive argument is denied despite the evidence to the contrary.
• Exclusion: Evidence which would change the outcome of an inductive argument is excluded from consideration.

Fallacies Involving Statistical Arguments

• Accident: Apply generalization when circumstances suggest that there should be an exception.
• Converse accident: Apply exception in circumstances where a generalization should apply.

Causal Fallacies

• Post hoc ergo propter hoc: Because one thing preceded another in time, it is held to cause the other.
• Joint effect: One thing is held to cause another when in fact they are both the joint effects of an underlying cause.
• Insignificant: One thing is held to cause another, and it does, but it is insignificant compared to other causes of the effect.
• Wrong direction: The direction between cause and effect is reversed.
• Complex cause: The cause identified is only a part of the entire cause of the effect.

A common form of this is the Rooster Syndrome — giving credit to the rooster crowing for the rising of the sun — but applied to giving credit or blame to leaders for events that occur on their watch to which they made little if any contribution.

Missing the Point

• Begging the question (petitio principii): The truth of the conclusion is assumed in the premises, or in hidden assumptions.
• Irrelevant conclusion: An argument in defense of one conclusion instead proves a different conclusion.
• Straw man: Attack an argument different from (and weaker than) the opposition's best argument.

Fallacies of Ambiguity

• Equivocation: Use same term with two or more different meanings.
• Amphiboly: Use sentence the structure of which allows two different interpretations.
• Accent: Emphasis on a word or phrase to suggest a meaning contrary to what the sentence actually says.

Category Errors

• Composition: Argue that because the attributes of the parts of a whole have a certain property, therefore the whole has that property.
• Division: Argue that because the whole has a certain property, therefore the parts have that property

Non Sequitur

• Affirming the consequent: Argument of the form: If A then B, B, therefore A.
• Denying the antecedent: Argument of the form: If A then B, Not A, thus Not B.
• Inconsistency: Assertion that contrary or contradictory statements are both true.

Syllogistic (Deductive) Errors

• Fallacy of four terms: Use a syllogism with four terms.
• Undistributed middle: Argue that two separate categories are connected because they share a common property.
• Illicit major: Reach conclusion with predicate about all of something when premises only mention some cases of the term in the predicate.
• Illicit minor: Reach conclusion with subject of the conclusion about all of something when premises only mention some cases of the term in the subject.
• Fallacy of exclusive premises: Use a syllogism with two negative premises.
• Affirmative conclusion from negative premise: Reverse the negation.
• Existential fallacy: Reach particular conclusion from universal premises that don't include an existence premise.
• Analogic "syllogism": Reasoning that A is similar to B, and B is similar to C, therefore A is similar to C. Of course, the relation of "similar" is not transitive, but if the target can be induced to presume it is, this ruse may succeed in persuading. This is a favorite method in the "informal reasoning" used by lawyers.

Fallacies of Explanation

• Subverted support: The phenomenon being explained doesn't exist.
• Non-support: Evidence for the phenomenon being explained is biased.
• Untestability: The theory which explains cannot be tested.
• Limited scope: The theory which explains can only explain one thing.
• Limited depth: The theory which explains does not appeal to underlying causes.

Fallacies of Definition

• Too broad: The definition includes items which should not be included.
• Too narrow: The definition does not include all the items which should be included.
• Failure to elucidate: The definition is more difficult to understand than the word or concept being defined.
• Circular definition: The definition includes the term being defined as a part of the definition.
• Conflicting conditions: The definition is self-contradictory, an oxymoron.

See also the page on Propaganda Techniques that discusses how these logical fallacies can be used.