Circular Logic

Attempting to prove a thesis by using an argument that derives from the thesis itself.
Using an argument that inherently implies the thesis one is attempting to prove is already true.

General form:
"A is true because B (which is true because A)"

Typical example:
"God exists because the Bible (which is the word of God) says so."

A valid logical argument must proceed from a clearly defined start (the premises) to clearly defined end (the thesis), and never the other way around. The premises of a thesis must be established beforehand, and can never be retroactively derived by something that derives from them, nor an argument can be retroactively used as a premise for itself; otherwise, any claim could be easily proven without any need for evidence.