conversio per accidens

logic

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June 24, 2026

Quick Summary

In logic, conversion per accidens is a method of transforming a proposition by switching its subject and predicate. This process is valid for universal affirmative propositions, where the original statement "all S is P" becomes "some P is S". This differs from simple conversion, which applies to universal negative and particular affirmative propositions and results in an equivalent statement without changing the quantity.12

The systematic study of logic, including concepts like conversion, was significantly advanced by Aristotle, who established rules and strategies for reasoning. His work, compiled in the Organon, laid the groundwork for understanding propositions and their interrelations.3

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history of logic
Categorical forms
…to be converted “accidentally” (per accidens). Propositions of form O cannot be converted at all; from the fact that some animal is not a dog, it does not follow that some dog is not an animal. Aristotle used these laws of conversion in later chapters of the Prior Analytics…
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…to be converted “accidentally” (per accidens). Propositions of form O cannot be converted at all; from the fact that some animal is not a dog, it does not follow that some dog is not an animal. Aristotle used these laws of conversion in later chapters of the Prior Analytics…
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syllogistic
…of inference is traditionally called conversio per accidens and holds as well in Eba implying Oab. In contrast, Oba neither implies nor is implied by Oab, and this is expressed by saying that O propositions do not convert. When a proposition is posed against the proposition that results from changing…
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…of inference is traditionally called conversio per accidens and holds as well in Eba implying Oab. In contrast, Oba neither implies nor is implied by Oab, and this is expressed by saying that O propositions do not convert. When a proposition is posed against the proposition that results from changing…
Read More