Circular reasoning

A form of logical fallacy in which the position that still has to be proven is already presupposed in a premise.

Take for example the following statement:

The Bible is God’s Word, for it is written [in the Bible] that “all Scripture is inspired by God”.

• Circular argument
• Zirkelschluss
• Circulus/-um in probando (Lat.: “circle in proving”)
• Circulus/-um in demonstrando (Lat.: “circle in the proof”)
• Idem per idem (Lat.: “the same by the same”)
• hýsteron próteron [Ὕστερον πρότερον] (grc.: “the later [is the] earlier”)

Occasionally, the term “petitio principii ” (“postulation of the beginning”) is used as a synonym for circular reasoning. However, the specific meaning of this expression is inconsistent and has changed over time. It is used here as a rhetoric figure that is analogous to circular reasoning.

The circular argument is closely related to the concept of a logical tautology, as for both can be said that when the premises are true it also holds true:

The sky is blue, because it is blue.

This also includes circular forms of Sorites arguments, in which the last step leads back to the starting point:

A₁ → A₂ → A₃ → … → Aₙ → A₁

However, circular reasoning covers also situation in which further assumptions may be made, which are not necessarily true. For an argument to be considered circular, it is enough if one of the premises already implies what is yet to be proven:

A ∧ B → A (from A and B follows A)

• Petitio principii
• Sorites
• Tautology

• Circular reasoning on Wikipedia