Begging the question, which goes by the technical name petitio principii, is a logical fallacy that occurs when an argument implicitly assumes its conclusion in a premise.
Consider the following argument:
- I know God exists because of all the good things He has done for me in my life.
Arranging this argument in the usual premise–conclusion sequence reveals the problem with it:
- God has done good things for me in my life.
- Therefore God exists.
The premise assumes that God exists in the first place, otherwise he couldn't "do" anything.