The classical rendition of the fallacy is as follows:
- Person A: "No Scotsman puts sugar on his porridge."
- Person B: "I am Scottish, and I put sugar on my porridge."
- Person A: "Well, no true Scotsman puts sugar on his porridge."
For example, a common claim is that Christians would never mass-murder people. Now if you bring up a counter-example ("this person was a devout Christian, and he mass-murdered people") the standard answer is the fallacy in its purest form: "He wasn't really a Christian."
This is basically a circular argument that's used to dodge any possible counter-arguments to the claim. The claim becomes basically impossible to prove wrong because any counter-example you may bring up will be countered with a simple "he/she wasn't really a Christian". It doesn't matter how deeply Christian the person might have been, it doesn't count if he was a mass-murderer.
The condition "does not commit mass-murder" (or any of the myriad other such claims) is basically implicitly added to the definition of "Christian", and thus anybody who does not fit that definition is thus "not really a Christian".
Ultimately this dodges the burden of proof for a claim such as "Christians never mass-murder people" (due to God acting upon them or such) because the definition of the term is basically circular.
Why is this a form of circular argumentation? Consider it like saying "no dog is white", and if a white dog is shown to you, just dismissing it by saying "that's not really a dog." And why isn't it a dog? Because it's white and, as established, "no dog is white", duh. The claim itself becomes the definition.