Petitio Principii (Begging the Question)
A logical fallacy where the conclusion is assumed in the premises, creating circular reasoning. The argument fails to provide independent support for its claim.
THE IDEA
4.1 PETITIO PRINCIPII
This fallacy is also known as begging the question. From the Latin, petitio principii is translated as “appealing to the first principles.” Petitio principii uses part of its conclusion in its own premise as support. This is a type of circular reasoning. Often this fallacy occurs when one uses two different words—one in the premises and one in the conclusion—which share the same definition. For example:
- Abortion is wrong because abortion is unethical.
In this example, the conclusion (abortion is wrong) is supported by the premise (abortion is unethical). However, this argument begs the question because an act that is unethical, by definition, is an act that is wrong. Consider these examples:
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I know that Gene and Susan slept together because I know that they had sex.
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Your honor, the defendant should be held responsible for stealing my client’s purse because he took something that did not belong to him.
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The Bible is true because it contains no falsehoods.
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Jan: Why did you decide to become a vegetarian, Anthony? Anthony: Because I don’t eat meat.
Most of these replies restate the claim instead of giving independent support. The purse example needs more context: evidence that someone took another person’s property is not automatically a restatement of a claim about responsibility. The question is whether the premise assumes the point under dispute.
When to use it
Analyzing political debates where opponents often use circular logic to justify their positions.
Evaluating legal arguments that assume the very point they are supposed to prove.
Teaching critical thinking to spot when someone is not providing genuine evidence.
Self-checking arguments in persuasive writing to ensure premises are independent of the conclusion.
FROM THEORY TO EVERYDAY LIFE
In practice
Example 1: Abortion is wrong because unethical
Premise: Abortion is unethical. Conclusion: Abortion is wrong. The premise and conclusion are synonymous, so no independent support is given.
Example 2: Sleeping together because had sex
Premise: Gene and Susan had sex. Conclusion: They slept together. 'Had sex' and 'slept together' are essentially the same, making the argument circular.
Example 3: Legal case – stealing purse
Premise: The defendant took something that did not belong to him. Conclusion: He should be held responsible for stealing. Whether this is circular depends on what the premise establishes and what is disputed; it is not automatically a restatement.
Example 4: Bible is true because no falsehoods
Premise: The Bible contains no falsehoods. Conclusion: The Bible is true. The premise assumes the very truth it aims to prove.
Example 5: Vegetarian because don’t eat meat
Jan asks why Anthony became a vegetarian. Anthony replies 'Because I don’t eat meat.' This merely restates the position without providing a reason.
A MOMENT TO REFLECT
Put it to work.
How does deductive validity differ from providing independent support for a disputed claim?
Think of a recent argument you encountered. Did it commit the petitio principii fallacy?
Why is circular reasoning considered a flaw in logic, and how does it undermine rational persuasion?
CONTINUE THE CONVERSATION
Source texts
- Informal Logical Fallacies A Brief Guide