it's circular because one can never have enough faith. A "mustard seed" is sufficient though.
Mark 9:23-25
New King James Version
23 Jesus said to him, “If[a] you can believe, all things are possible to him who believes.”
24 Immediately the father of the child cried out and said with tears, “Lord, I believe; help my unbelief!”
25 When Jesus saw that the people came running together, He rebuked the unclean spirit, saying to it, “Deaf and dumb spirit, I command you, come out of him and enter him no more!”