Ad Hominem: Poisoning the Well





This fallacy occurs when an attempt is made to discredit what a person might later claim by presenting unfavorable information (true or not) about the person. It is part of the Ad Homimen family and can be looked at as a pre-emptive Ad Hominem. The reasoning has the following form:


Premise 1: Unfavorable information (true or false) about person A is presented.

Conclusion: Therefore, future claims made by person A will be false.


This is poor reasoning for the same reason that all Ad Hominem fallacies are fallacies: attacking a person does not refute their claims (or arguments), whether they have already been made or will be made in the future. The following silly example illustrates the bad reasoning:


Sam: “Donald has no ethics and lies even when the truth would serve him better. You just wait, the next thing he says will be a lie.”

Mark: “That makes sense.”

Donald: “I was talking to this smart guy the other day and he said triangles have three sides. Always. You know what, he is right. They always do!”

Mark: “Aha, triangles do not have three sides! I knew that geometry teacher was a liar!”


The person making the attack hopes that the unfavorable information will bias listeners against the target and that they will reject claims they might make. In most cases, the attack will be aimed at a category of claims the person might make rather than anything they might happen to say. For example, a Poisoning the Well attack on a judge might focus on what they will say in an upcoming ruling.

As with the other Ad Hominems, this fallacy can have considerably psychological force but has no logical force. It is easy to mistake Poisoning the Well for other Ad Hominems because it will often duplicate these other fallacies with one critical difference. Poisoning the Well aims at future claims rather than claims that have been made. A person can, of course, combine fallacies to attack claims that have been made and claims that will be made.

Being, in effect, a pre-emptive Ad Hominem, this fallacy is often used when it is not known for sure what the person will say. For example, a bad faith debater who is speaking first might use this fallacy against their opponent. It is also often used in cases in which the target is unable to reply in real time. For example, a pundit might use this fallacy in their YouTube video or in their TV broadcast.

This fallacy can be effective for the same reasons that other Ad Hominems can be effective. It also can gain the advantage of being a pre-emptive attack. If used effectively against a target, they will start at a disadvantage in that they will need to overcome the pre-emptive attack before making their positive case. People are also sometimes inclined to believe the first thing they hear, especially if it is something negative.

Reasonable criticisms of a person’s credibility might be mistaken for Poisoning the Well (or another Ad Hominem). Properly challenging a person’s credibility involves raising reasonable concerns that are relevant to the reliability and accuracy of their claims. The Appeal to Authority includes a discussion of some factors relevant to a person’s credibility. Proper credibility challenges also do not include the inference that a person’s claim is false simply because of the challenge to their credibility.


Defense: As with its fellow Ad Hominems, the main defense against this fallacy is remembering that an attack on a person does not refute their claims (or arguments).


Example #1:

“Don’t listen to him, he’s a scoundrel.”


Example #2:

“Before turning the floor over to my opponent, I ask you to remember that those who oppose my plans do not have the best wishes of the university at heart.”


Example #3:

Sally: “Eric is such a decadent wastrel.”

Ann: “A what?”

Sally: “A good-for-nothing. A wasteful person. Eric is also decadent.”

Ann: “He sounds awful.”

Sally: “He is. But he also has a certain charm. But don’t listen to him, especially about politics. Everything that comes out of his mouth is a lie.”

Eric: “Hello, ladies. I was just discussing that bill about reducing regulations on businesses. Such a good idea!”

Ann: “Humph. I think that is a terrible idea.”

Eric: “Why? Will you listen to my reasons?”

Ann: “Get away you decedent weasel!”

Eric: “What?”


Example #4

Before class

Bill: “Boy, that professor is a real jerk. I think he is some sort of Eurocentric fascist.”

Jill: “Yeah.”

During Class:

Prof. Jones: “…and so we see that there was never any ‘Golden Age of Matriarchy’ in the ancient world.”

After Class:

Bill: “See what I mean?”

Jill: “Yeah. There must have been a Golden Age of Matriarchy, since that jerk said there wasn’t.”

Originally appeared on A Philosopher’s Blog Read More