Quora Answer: What is an Ad Hominem Attack?

The following is my answer to a Quora question: “What is an ad hominem attack?” 

“Ad hominem” is Latin, and literally means “to the person”.  It is a shortened form of “argumentum ad hominem”, “argument to the person”.  In rhetoric, it is an argument, where the speaker attacks the integrity, or a character attribute of the person, instead of the contentions of his argument for or against a position.  There are two kinds of argumentum ad hominem: fallacious and valid. 

A fallacy in rhetoric involving argumentum ad hominem is to directly attack the person to discredit his argument even though his character is not central to the contention he has raised.  This is often employed when there is no strong argument against his contention.  It is emotive, and not based on logic.  For example, someone states that climate change is happening, and cites scientific sources.  Instead of addressing those sources, the opposing speaker questions his ability to tell the truth. 

A valid argumentum ad hominem is to attack the credibility of the person when the credibility of his testimony is predicated on his character.  This is often used in court, to question the character of a witness, to establish or dismiss testimony.