False Conversion
False Conversion is a formal fallacy where the subject and predicate of a proposition are incorrectly switched in the conclusion, specifically for propositions of type 'A' (all) or type 'O' (some...not).
THE IDEA
False Conversion
Also known as: illicit conversion
Description: The formal fallacy where the subject and predicate terms of the proposition are switched (conversion) in the conclusion, in a proposition that uses “all” in its premise (type “A” forms), or “some/not” (type “O” forms).
Logical Form:
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Premise: All A are B.
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Conclusion: All B are A.
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Premise: Some A are not B.
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Conclusion: Some B are not A.
Examples:
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Premise: All Hollywood Squares contestants are bad actors. Conclusion: Therefore, all bad actors are Hollywood Squares contestants.
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Premise: Some politicians are not liars. Conclusion: Therefore, some liars are not politicians.
Explanation: It does not follow logically that just because all Hollywood Squares contestants are bad actors, that all bad actors actually make it on Hollywood Squares. Same form problem with the second example – but we used “some” and “are not”.
Exception: None. But remember that type “E” and type “I” forms can use conversion and remain valid.
When to use it
Analyzing the logical structure of categorical arguments in philosophy or logic courses.
Evaluating political speeches that make sweeping generalizations about groups.
Checking your own reasoning when drawing conclusions from statistical data.
Teaching students to identify formal fallacies in everyday reasoning.
FROM THEORY TO EVERYDAY LIFE
In practice
Hollywood Squares
Premise: All Hollywood Squares contestants are bad actors. Conclusion: Therefore, all bad actors are Hollywood Squares contestants.
Politicians and liars
Premise: Some politicians are not liars. Conclusion: Therefore, some liars are not politicians.
A MOMENT TO REFLECT
Put it to work.
Can you think of a real-world argument you've encountered that commits the false conversion fallacy?
Why does illicit conversion seem plausible even though it is logically invalid?
How can you mentally test a categorical syllogism to avoid incorrectly switching terms?
CONTINUE THE CONVERSATION
Source texts
- Logically Fallacious - The Ultimate Collection of Over 300 Logical Fallacies