Circular reasoning

Core claim: A formal fallacy in which the conclusion that is supposed to be proven is already presupposed in one of the premises, rendering the argument epistemically empty.

• The premise entails or restates the conclusion (e.g., “A because A” or “A ∧ B → A”).
• A chain of implications eventually loops back to its starting point (A₁ → A₂ → … → Aₙ → A₁).
• The only support offered for a claim is another statement whose truth depends on the original claim.

• Circular reasoning is closely related to logical tautology: both are formally valid but provide no new information. However, not every tautology is fallacious – it becomes so when presented as a proof.
• The term “petitio principii” is sometimes used synonymously, but on this site it refers specifically to the relevance-framed version of the same pattern.
• Avoid using “circulus vitiosus” or “vicious circle” for this fallacy – those terms mainly refer to negative feedback systems.

• The premise provides independent support for the conclusion, even if both happen to be true.
• A statement is merely repeated for emphasis without claiming to justify itself (e.g. “A rose is a rose”).
• The argument is a legitimate definition or identity claim (e.g., “A bachelor is an unmarried man”).

• Petitio principii (MD) – the same underlying pattern framed as a relevance fallacy (“begging the question”). Ask: is the focus on formal structure (→ circular reasoning) or on the argument's failure to engage with what needs proving (→ petitio principii)?
• Tautology (MD) – a statement true by logical form; only fallacious when posed as proof. Ask: is this presented as an argument (→ circular), or merely stated (→ tautology)?

Input: “This document is legally binding, for it is written in Section 4 of this very document that all its clauses are legally binding.”
Output: The authority of the document rests on the document itself. This reasoning is circular, if no additional independent justification for the claim is given.

Input: “The server clock is absolutely accurate, because it always matches this PC's clock perfectly.” (While the PC clock is actually synchronized with the server)
Output: The PC clock is not an independent reference – it is itself synchronised with the server. Using it to validate the server clock is therefore circular: the system is being checked against its own output. Use a truly independent time source instead.

Input: “The AI confirmed that this term is used in this sense, so we can be confident the usage is correct.” (where the AI was trained on the very source being verified)
Output: If the AI learned its knowledge from the same source you're trying to verify against, the confirmation is circular: the source “proves” itself through the AI as intermediary. Verify with independent sources instead.

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