“An ad hominem attack against an intellectual, not against an idea, is highly flattering. It indicates that the person does not have anything intelligent to say about your message.”

― Nassim Nicholas Taleb

Responding to an argument by attacking the person instead of critiquing their argument is the ad hominem logical fallacy. Ad hominem is a form of red herring fallacy which occurs when someone introduces irrelevant information to distract from the original topic or argument. There are three main forms of the ad hominem fallacy: abusive, circumstantial and tu quoque.

Suppose a theist makes the argument below. Notice the response from each form of ad hominem.

Argument: Believing in things you can't justify is illogical. Philosophical atheists cannot justify their belief that God is imaginary. Therefore atheists are illogical.

Ad Hominem Abusive
-the person's character is attacked instead of their argument. (Justifiably pointing out a person's character flaw is not a fallacy though.)

Response: God you're stupid!

Ad Hominem Circumstantial
-the person's circumstances are attacked instead of their argument.

Response: You're one to talk, you believe in a magical sky daddy!

Ad hominem Tu Quoque (appeal to hypocrisy)
-instead of dealing with the argument, a person attacks the arguer for being hypocritical.

Response: What a hypocrite. That post you made yesterday was illogical.