21 Common Logic Errors

1. Ad Hominem Argument An argument that counters another’s claim or conclusion by attacking the person, rather than attacking the argument itself. (Latin, “against the man.”) A specific example of the Genetic Fallacy which assumes that an idea is not true because of its origin. E.g., a Democrat (or Republican) has a idea, therefore, it must be bad. (Also called the Fallacy of Irrelevance.)

2. Argument from Antiquity An argument is true because it has been held for a long time. Related to the Argument from Numbers, the argument that because many people think something is true, it is true (also known as the Argument Ad Populum).

3. Argument from Authority Stating that a claim is true because an authority (person or group of people) says it is true.

4. Argument from Final Consequences A claim is true because of a purpose or outcome that is served (or vice versa). Also known as a Teleological Argument. E.g., “evolution cannot be true because accepting it will lead to immorality.”

5. Argument from Ignorance The claim that a specific belief is true because we don’t know that it isn’t true.

6. Argument from Personal Incredulity Because you, personally, cannot understand or accept a proposition it is, therefore, not true. Often coupled with a False Dichotomy (see below) as in “I don’t see how the eye could have evolved, therefore, God did it.”

7. Begging the Question or a Tautology A statement that hinges on A=A (or A=B therefore B=A), which is simply restating the premise. For example, “he is unintelligent because he is stupid” or “only a criminal would commit a crime; the fact that criminals commit crimes is proof of this.” (Tautology, literally a repetition.) The “proof” is a restatement of the premise.

8. Confirmation Bias Noticing only the facts that support your thesis but ignoring those that do not.

9. Confusing Correlation with Causation Assuming cause and effect for two variables because they are correlated. E.g., “heroin addicts drank milk as a child, therefore, milk causes heroin addiction.” This is similar to the Post-hoc Ergo Propter Hoc fallacy (see below).

10. Confounding the Unexplained with the Unexplainable Because we do not currently have an explanation for a phenomenon does not mean that it is forever unexplainable or that it requires a paranormal/supernatural explanation.

11. False Continuum The idea that because there is no definitive demarcation between two extremes, the distinction between the extremes is not real or meaningful. E.g, because there is a fuzzy line between science and pseudoscience they are the same thing. Related to the Slippery Slope (see below).

12. False Dichotomy Arbitrarily reducing a set of possibilities to only two.

13. False Premise An incorrect/untrue underlying assumption. Often found in an argument that is otherwise logically consistent but which leads to a false conclusion.

14. Inconsistency Applying specific criteria or rules to one belief, claim, argument, or position but not to others.

15. Moving the Goalpost A method of denial by arbitrarily moving the criteria for proof, acceptance, or rejection out of the range of whatever evidence currently exists or is agreed upon.

16. Non-Sequitur This refers to an argument in which the conclusion does not follow from the premise. In other words, a logical connection is implied where none exists. (Latin, “doesn’t follow.”). Really, all logical fallacies are non-sequiturs.

17. Post-hoc Ergo Propter Hoc This fallacy follows the basic form of: A preceded B, therefore A caused B. This argument assumes cause and effect for two events just because they are temporally related. (Latin, “after this, therefore because of this.”)

18. Slippery Slope The argument that a position is not acceptable because accepting the position means that the extreme of the position must also be accepted. But, moderate positions do not necessarily lead down a slippery slope to the extreme. Careful here: Reductio Ad Absurdum (Latin: "reduction to the absurd"), a form of argument in which a proposition is disproved by logically following its implications to an absurd conclusion, can be a valid argument.

19. Straw Man Arguing against a misrepresentation or over-simplification of the position actually held by an opponent.

20. Special Pleading (Ad Hoc Reasoning) The arbitrary introduction of new elements into an argument in order to amend them so that they appear valid. E.g., “ESP doesn’t work in the presence of skeptics.” A subtle fallacy which is often difficult to recognize.

21. Tu Quoque To reject a position because someone (inconsistently) holds it. Also called an Appeal to Hypocrisy. Person 1: “Smoking tobacco is bad for you.” Person 2: “You smoke, therefore, your argument is invalid.” (Latin, “you too.”)