No True Scotsman

The No True Scotsman (NTS) fallacy is a logical fallacy that occurs when a debater defines a group such that every groupmember possess some quality. For example, it is common to argue that “all members of [my religion] are fundamentally good”, and then to abandon all bad individuals as “not true [my-religion]-people”. This can occur in two ways:

  1. During argument, someone re-defines the group in order to exclude counter-examples. Instead of backing down from “all groupmembers are X” to “most groupmembers are X”, the debater simply redefines the group.[2][3]
  2. Before argument, someone preemptively defines some group such that the group definitionally must be entirely “good” or entirely “bad”. However, this definition was created arbitrarily for this defensive purpose, rather than based on the actual qualities of the group.[4]

NTS can be thought of as a form of inverted cherry picking, where instead of selecting favourable examples, you reject unfavourable ones. The NTS fallacy paves the path to other logical fallacies, such as letting the “best” member of a group represent it. Thanks to these remarkable qualities, the NTS fallacy is a vital tool in the promotion of denialism.

In short: Both arguers should agree on a definition and stick to it.

This is an extract from “No True Scotsman” at RationalWiki which is licensed under CC BY-SA 3.0.