Definition
Circular reasoning occurs when an argument's chain of support eventually loops back to depend on the very conclusion it is trying to establish. Stated plainly, it takes the shape "A is true because of B, and B is true because of A." Begging the question is the most common single-step version of this pattern; circular reasoning describes the broader family, including longer loops that pass through several claims before returning to the start.
Example
"This source is reliable because it accurately reports the facts, and we know it accurately reports the facts because it is a reliable source." Each half of that sentence depends entirely on the other one being true first. Neither is independently established, so together they establish nothing.
Common mistakes
Circular reasoning can be hard to spot when the loop runs through several intermediate claims instead of just two, especially across a long speech where the connection is not stated back to back. A practical way to check is to map each claim in a chain to what it actually rests on. If following that chain far enough eventually leads back to the original claim, the argument is circular and needs an external starting point, some piece of evidence or reasoning that does not depend on the conclusion being true.