Explanation
Petitio principii is a fallacy in which the proposition to be justified is already assumed, explicitly or covertly, in the premises. The inference may look formally valid but supplies no independent support: saying a rule is right because it is a correct rule merely moves within the starting claim. More complex cases conceal circularity through synonyms, intermediate steps, or contested background assumptions. Not every use of an assumption within a larger coherent system is automatically fallacious; what matters is whether it is being justified to a doubter by reasons that already require its truth. Petitio is also not identical with tautology. The movement follows an argument apparently outwards and discovers that its justificatory path returns unnoticed to the starting point without acquiring additional evidence.
Movement of thought
1. A disputed claim is first presented as a conclusion and calls for reasons that should also carry weight for a doubter. 2. The argument introduces premises or definitions whose plausibility already depends on the truth of that same disputed claim. 3. Several verbal reformulations may conceal the circle and create the impression of an advancing proof. 4. A counter-test reconstructs dependencies and asks whether any premise provides independent support, separating petitio from valid inference and tautology. 5. The reasoning returns to its beginning without epistemic gain: the claim has been repeated or assumed rather than justified to a doubter.
Scientific use case
Argumentation theory, logic, philosophy of science, and discourse analysis reconstruct premise dependencies, disputed definitions, and dialectical starting points. Argument maps expose indirect circularity.
Example
“This source is reliable because all its reports are true; we know the reports are true because they come from this reliable source.” The reasons support one another only circularly.
Counterexample
A new conclusion is derived from generally accepted measurements and an independently tested rule. The fact that a valid conclusion is logically contained in its premises does not make the inference question-begging.
Distinction
Distinguish it from tautology, formally valid deduction, circular reasoning as a broader category, equivocation, enthymeme, definition, justificatory regress, and repetition; show dialectical disputedness and premise dependence.
Effect on thought and perception
Petitio can make an unsupported position appear self-evident through repetition and block genuine dispute; exposing it redirects inquiry towards independent evidence or shared premises.
Resonance
The argument seems to travel, yet its line closes around its own starting point; within the circle, only the claim answers itself.
Cite this entry
Reich, Michael (Hrsg./ed.): „Petitio principii“. Atlas der Sprache, AT-088-021. ATLAS 260, data model 1.6.0. https://atlasoflanguage.com/concept/AT-088-021The address remains stable; editorial content may evolve. Editorial reading state: 2026-10-15. Include your access date for precise references.
Historical background
Mill’s account of petitio principii is not restricted to the blatant repetition of a conclusion as its own premise. He considers how people can retain confidence in a belief while forgetting the grounds on which they accepted it. The belief may then be used to support the very proposition on which its own warrant depends. His discussion, in dialogue with Whately, connects the formal defect with an ordinary difficulty in tracing evidence. Circular reasoning is a stronger, explicit form in which propositions are offered as reciprocal proofs. Mill also warns against confusing the internal coherence of an explanatory system with its external support. For the Atlas, this makes the countertest more demanding than searching for identical sentences: can the supposed supporting premise be justified independently in the disputed context? A chain of individually elegant formulations may still return to its starting assumption. This historical source supplies an English discussion using both the Latin label and begging the question, not proof of either expression’s first use. It also leaves room for distinguishing an independently supported premise from one whose only available support is the contested conclusion.
How conceptual history changes the analysis
Logical consequence is not itself circularity. The defect is lack of independent warrant for a premise on which the argument depends.
References for the historical section
- John Stuart Mill, A System of Logic, Ratiocinative and Inductive, eighth edition, New York: Harper & Brothers, 1882; Project Gutenberg ebook 27942 · Book V, Chapter VII, §2: begging the question and reasoning in a circle
Selected named passages read in the transcription. The title page identifies an 1882 printing of the eighth edition, not the first edition. No claim that these passages were present unchanged in the first edition; no first-attestation or exhaustive reception claim.