Circular reasoning (Latin: circulus in probando, “circle in proving”; also known as circular logic) is a logical fallacy in which the reasoner begins with what they are trying to end with. The components of a circular argument are often logically valid because if the premises are true, the conclusion must be true.

Circular reasoning is not a formal logical fallacy but a pragmatic defect in an argument whereby the premises are just as much in need of proof or evidence as the conclusion, and as a consequence the argument fails to persuade. Other ways to express this are that there is no reason to accept the premises unless one already believes the conclusion, or that the premises provide no independent ground or evidence for the conclusion. Begging the questionis closely related to circular reasoning, and in modern usage the two generally refer to the same thing.

Circular reasoning is often of the form: “A is true because B is true; B is true because A is true.” Circularity can be difficult to detect if it involves a longer chain of propositions. Academic Douglas Walton used the following example of a fallacious circular argument:

Wellington is in New Zealand.
Therefore, Wellington is in New Zealand.

He notes that, although the argument is deductively valid, it cannot prove that Wellington is in New Zealand because it contains no evidence that is distinct from the conclusion. The context – that of an argument – means that the proposition does not meet the requirement of proving the statement; thus, it is a fallacy. He proposes that the context of a dialogue determines whether a circular argument is fallacious: if it forms part of an argument, then it is. Citing Cederblom and Paulsen 1986:109, Hugh G. Gauch observes that non-logical facts can be difficult to capture formally:

“Whatever is less dense than water will float, because whatever is less dense than water will float” sounds stupid, but “Whatever is less dense than water will float, because such objects won’t sink in water” might pass.

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