Argument from authority. A said X, A is an authority on X, Therefore, X is true. This is a valid inductive argument. It is valid when A is an actual authority

Argument from authority. A said X, A is an authority on X, Therefore, X is true. This is a valid inductive argument. It is valid when A is an actual authority, when they are speaking truthfully, etc. It doesn't guarantee the truth of the conclusion but it is a fine argument. How do you know Booth shot Lincoln? You read it in a textbook, the textbook is written by someone who is an authority on history or that part f history, etc. It isn't deductive (like a proof in geometry or logic) but it is a good argument. Many logic books call argument from authority invalid and what they mean is that it isn't deductively valid but it is an inductively valid argument form (If it were always invalid, we would be quite mired in ignorance).