Circular reasoning (Latin: circulus in probando, "circle in proving"; also known as paradoxical thinking or 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. However, the argument is useless because the conclusion is one of the premises. Circular logic cannot prove a conclusion because, if the conclusion is doubted, the premise which leads to it will also be doubted. Begging the question is a form of circular reasoning.
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.
Circular reasoning and 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".
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