The poisoning the well fallacy is a particular form of ad hominem attack, whereby the arguer seeks to discredit a person from a debate or conversation instead of addressing that person's actual opinions or arguments. It is so-named because once people believe a well to be poisoned, they obviously will not want to drink any water from that well. Similarly, those who commit this fallacy hope to make their audience shy away from any contributions to the discussion made by the targeted individual (or group of individuals).

Here is an example:

A reporter begins to ask the spokesperson of an airplane manufacturer a question at a press conference.

Reporter: "Can you address the comments made recently by Mr. Smith about safety concerns on your new aircraft, specifically..."

Spokesperson: "Let me stop you right there. Mr. Smith has never designed or flown this type of aircraft, so you can disregard whatever he has to say. Next question."

In the classic case of this fallacy, the arguer "poisons the well" before the target's comments can even be aired. However, the fallacy can also be committed retroactively after someone has already participated in the debate, in order to encourage the audience to dismiss all of that person's past, present and future comments about the topic.

It is also poisoning the well when, instead of the target being a particular person, the target is just anyone who dares take an opposing position, such as in this example:

A: "Can you reply to the concerns about whether last year's Bill really reduced homelessness?"

B: "Anyone who thinks it didn't, doesn't know the first thing about the data on homelessness."

Arguer B seeks to automatically discredit anyone who disagrees, because a presumed defect.

It is a fallacy because, regardless of who made a comment, we cannot be certain about the truth (or falsity) of that comment except by examining the content of the comment itself.

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