(also known as: discrediting, smear tactics)
Description: To commit a preemptive ad hominem attack against an opponent. That is, to prime the audience with adverse information about the opponent from the start, in an attempt to make your claim more acceptable, or discount the credibility of your opponent’s claim.
Adverse information (be it true or false) about person 1 is presented.
Therefore, the claim(s) of person 1 will be false.
Tim: Boss, you heard my side of the story why I think Bill should be fired and not me. Now, I am sure Bill is going to come to you with some pathetic attempt to weasel out of this lie that he has created.
Explanation: Tim is poisoning the well by priming his boss by attacking Bill’s character, and setting up any defense Bill might present as “pathetic”. Tim is committing the fallacy here, but if the boss were to accept Tim’s advice about Bill, she, too, would be committing the fallacy.
I hope I presented my argument clearly. Now, my opponent will attempt to refute my argument by his own fallacious, incoherent, illogical version of history.
Explanation: Not a very nice setup for the opponent. As an audience member, if you allow any of this “poison” to affect how you evaluate the opponent’s argument, you are guilty of fallacious reasoning.
Exception: Remember that if a person states facts relevant to the argument, it is not an ad hominem attack. In the first example, if the other “poison” were left out, no fallacy would be committed.
Tim: Boss, you heard my side of the story why I think Bill should be fired and not me. Now, I am sure Bill is going to come to you with his side of the story, but please keep in mind that we have two witnesses to the event who both agree that Bill was the one who told the client that she had ugly children.