Petitio principii is the logical fallacy of circular reasoning. It’s when you use the point to be proved as a premise in the argument to prove it. It’s a phrase from the Latin that literally means assuming the point.
Interestingly, Petitio principii doesn’t result in an argument that isn’t true, just one that is misleading. We expect arguments to go along these lines:
P is true,
If P is true, Q is true
If Q is true, R is true
Therefore R is true
An argument with circular reasoning can go along these lines:
P is true,
If P is true, Q is true
If Q is true, R is true
If R is true, P is true
Therefore P is true
The conclusion in this case is indeed true, but the argument reduces to a tautology: P is true, therefore P is true.
(Source: philosophy.lander.edu)
