Fallacy of four terms
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The fallacy of four terms (Latin: quaternio terminorum) is the logical fallacy that occurs when a categorical syllogism has four terms.
Valid categorical syllogisms always have three terms:
- Major premise: All fish have fins.
- Minor premise: All goldfish are fish.
- Conclusion: All goldfish have fins.
Here, the three terms are: "goldfish," "fish," and "fins."
Using four terms invalidates the syllogism:
- Major premise: All fish have fins.
- Minor premise: All goldfish are fish.
- Conclusion: All humans have fins.
The premises don't connect "humans" with "fins", so the reasoning is invalid. Notice that there are four terms: "fish", "fins", "goldfish" and "humans". Two premises aren't enough to connect four different terms, since in order to establish connection, there must be one term common to both premises.
In everyday reasoning, the fallacy of four terms occurs most frequently by equivocation: using the same word or phrase but with a different meaning each time, creating a fourth term even though only three distinct words are used:
- Major premise: Nothing is better than complete happiness.
- Minor premise: A ham sandwich is better than nothing.
- Conclusion: A ham sandwich is better than complete happiness.
The fallacy of four terms is a syllogistic fallacy. Types of syllogism to which it applies include statistical syllogism, hypothetical syllogism, and categorical syllogism, all of which must have exactly three terms.
| Argument from fallacy | Fallacy of modal logic | Masked man fallacy | Appeal to probability
|
|
|---|---|
| Fallacy of propositional logic: | |
| Affirming a disjunct | Affirming the consequent | Commutation of Conditionals Denying a conjunct | Denying the antecedent | Improper Transition |
|
| Fallacy of quantificational logic: | |
| Existential fallacy | Illicit Conversion | Quantifier shift | Unwarranted contrast | |
| Syllogistic fallacy: | |
| Affirmative conclusion from a negative premise | Negative conclusion from an affirmative premise Exclusive premisses | Necessity | Four-term Fallacy | Illicit major | Illicit minor | Undistributed middle |
|
| Other types of fallacy | |
[edit] External links
- Stephen's guide: Fallacy of four terms
- Fallacy files: Fallacy of four terms
- Atheismweb: Fallacy of four terms
- http://onegoodmove.org/fallacy/four.htm Fallacy of the Four Terms (quaternio terminorum)]
