No true Scotsman or appeal to purity is an informal fallacy in which one attempts to protect an initial a posteriori claim from a subsequent falsifying counterexample by then covertly modifying the initial claim. Rather than admitting error or providing evidence that would disqualify the falsifying counterexample, the claim is modified into an a priori claim to definitionally (as opposed to evidentially) exclude the undesirable counterexample. The modification is usually identifiable by the use of non-substantive rhetoric such as "true", "pure", "genuine", "authentic", or "real", which can be used to locate when the shift in meaning of the claim occurs.
Philosophy professor Bradley Dowden explains the fallacy as an "ad hoc rescue" of a refuted generalization attempt. The following is a simplified rendition of the fallacy:
Occurrence
The "no true Scotsman" fallacy is committed when the arguer satisfies the following conditions:
not publicly retreating from the initial, falsified a posteriori assertion
offering a modified assertion that definitionally excludes a targeted unwanted counterexample...
Evan Prodromou
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