Circular Reasoning

Circular reasoning occurs when the reasoner begins with what he or she is trying to end up with. The most well known examples are cases of the fallacy of begging the question. However, if the circle is very much larger, including a wide variety of claims and a large set of related concepts, then the circular reasoning can be informative and so is not considered to be fallacious. For example, a dictionary contains a large circle of definitions that use words which are defined in terms of other words that are also defined in the dictionary. Because the dictionary is so informative, it is not considered as a whole to be fallacious. However, a small circle of definitions is considered to be fallacious.

Example:

Definition: A couch is a sofa.
Definition: A sofa is a davenport.
Definition: A davenport is a couch.

For additional difficulties in deciding whether an argument is deficient because it is circular, see Begging the Question.