A circular argument is technically a valid argument. For every case in which the premises are true, the conclusion will be true. So what makes it a bad one?

 

Thanks for your question. Although we tend to focus on validity in logic, it is actually only the minimal baseline requirement for a “good” argument. Valid means that the conclusion is guaranteed to be true if the premises are true. But in order for an argument to be “sound” it needs also to have true premises. Otherwise, it might just be “vacuously valid” as in the case where the premises contradict each other, and thus can never be true simultaneously.

P and not P
Therefore X

is also a “valid” argument, no matter what P and X might be. But it can never be sound.
Your case, however is different.

P
Therefore P

The problem with the circular argument is not that it cannot be sound, but that it does not play the function of an argument –to convince us of things we did not believe before. A circular argument yields exactly and only what you put into it. It does not increase your understanding or store of knowledge. It is “bad” not because it is invalid, but because it is non-functional. If presented in a debate or philosophical paper, it is considered misleading and illegitimate because it claims to prove something that it presents as a given or as an assumption.