\begin{definition}[Definition:Circular Proof]
A '''circular proof''' is a proof $A$ in terms of another proof $B$ such that the truth value of $B$ depends (either directly or indirectly) on the truth value of $A$.
A '''circular proof''' is valid, as every statement implies itself. 
The problem of a '''circular proof''' is that if the truth of a conclusion is [http://en.wiktionary.org/wiki/dubious dubious], premises that are equally dubious provide no reason to support the conclusion.
\end{definition}