Begging the Question

Petitio Principii

In Logic, Begging the Question is an informal fallacy of presumption. This fallacy occurs when someone creates the illusion that a conclusion is warranted by assuming the truth of an unstated premise necessary for reaching the conclusion.

For example, “Murder is wrong. Therefore, abortion is always wrong.”

In this case the premise is true by definition, but we have assumed that abortion is a type of murder. This unstated premise is assumed to be true (which it may or may not be), and is necessary in order to warrant the conclusion that abortion is wrong. So, we have assumed what we needed to demonstrate before we can draw our conclusion.