Begging the question

In classical rhetoric and logic, begging the question or assuming the conclusion (Latin: petītiō principiī) is an informal fallacy that occurs when an argument's premises assume the truth of the conclusion. Historically, begging the question refers to a fault in a dialectical argument in which the speaker assumes some premise that has not been demonstrated to be true. In modern usage, it has come to refer to an argument in which the premises assume the conclusion without supporting it. This makes it more or less synonymous with circular reasoning.

Some examples are:

• "People have known for thousands of years that the earth is round. Therefore, the earth is round."
• "Drugs are illegal so they must be bad for you. Therefore, we ought not legalize drugs because drugs are bad for you."

Informal misuse of the phrase "begs the question" also occurs with an entirely dissimilar sense in place of "prompts a question" or "raises a question".