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 question is 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:
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.
The problem of induction
Joel Feinberg and Russ Shafer-Landau note that "using the scientific method to judge the scientific method is circular reasoning". Scientists attempt to discover the laws of nature and to predict what will happen in the future, based on those laws. However, per David Hume's problem of induction, science cannot be proven inductively by empirical evidence, and thus science cannot be proven scientifically. An appeal to a principle of the uniformity of nature would be required to deductively necessitate the continued accuracy of predictions based on laws that have only succeeded in generalizing past observations. But as Bertrand Russell observed, "The method of 'postulating' what we want has many advantages; they are the same as the advantages of theft over honest toil".
- Dowden, Bradley (27 March 2003). "Fallacies". Internet Encyclopedia of Philosophy. Retrieved April 5, 2012.
- Nolt, John Eric; Rohatyn, Dennis; Varzi, Achille (1998). Schaum's outline of theory and problems of logic. McGraw-Hill Professional. p. 205. ISBN 9780070466494.
- Walton, Douglas (2008). Informal Logic: A Pragmatic Approach. Cambridge University Press. ISBN 9780521886178.
- Walton, Douglas (1992). Plausible argument in everyday conversation. SUNY Press. pp. 206–207. ISBN 9780791411575.
- Gauch, Hugh G. (2003). Scientific Method in Practice. Cambridge University Press. p. 184. ISBN 9780521017084. LCCN 2002022271.
- Feinberg, Joel; Shafer-Landau, Russ (2008). Reason and responsibility: readings in some basic problems of philosophy. Cengage Learning. pp. 257–258. ISBN 9780495094920.