Logical Fallacy of the Week, Week 13: Fallacy of Four Terms

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The fallacy of four terms

(Latin: quaternio terminorum) is the logical fallacy that occurs when a syllogism has four (or more) terms rather than the requisite three. This form of argument is thus invalid.

Contents
1 Explanation
2 Reducing terms
3 Classification
4 References
5 External links

Explanation - 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.

Please follow the link to the rest of the story. RK