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Definist Fallacy

(also known as: persuasive definition fallacy, redefinition)

Description: Defining a term in such a way that makes one’s position much easier to defend.

Logical Form:

A has definition X.

X is harmful to my argument.

Therefore, A has definition Y.

Example #1:

Before we argue about the truth of creationism, let’s define creationism as, “The acceptance of a set of beliefs even more ridiculous than those of flat-earthers.”

Example #2:

Before we argue about the truth of creationism, let’s define evolution as, “Faith in a crackpot theory that is impossible to prove with certainty.”

Explanation: It should be clear by the two examples who is defending what position.  Both arguers are taking the opportunity to define a term as a way to take a cheap shot at the opponent.  In some cases, they might actually hope their definition is accepted, which would make it very easy to defend, compared to the actual definition.

Exception: When a definition used is really an accurate definition from credible sources, regardless of the damage it might do to a position.

Tip: Do not accept definitions put forth by the opponent unless you researched your definition on your own, and agree.


Bunnin, N., & Yu, J. (2008). The Blackwell Dictionary of Western Philosophy. John Wiley & Sons.

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