There is a debating maneuver called the "No-true-Scotsman" maneuver. Some refer to it as a logical fallacy. It goes like this:
- The speaker says: "No true Scotsman drinks wine."
- His interlocutor says: "This cannot be so. I know a guy name Ian MacGregor; he was born and raised in Edinburgh; we had dinner the other night, and he drank wine."
- The speaker then says: "Well, he may have a Scottish name, and have been born and raised in Scotland, but he nonetheless cannot be a "true" Scotsman.
- The interlocutor asks: "How can it be that a Scotsman, born and raised in Scotland, is not a "true" Scotsman?"
- The speaker replies: "Because you said he drinks wine, and no "true Scotsman" drinks wine.
What happened is that the speaker had a private (or "stipulative" or "stipulatory") definition of "true Scotsman" that he did not share with his interlocutor before he started the discussion. The interlocutor reasonably assumed he and the speaker were using plain English, and that words they were using meant what those words usually mean. In plain English, a Scotsman born and raised in Scotland is certainly a "true Scotsman." The speaker allowed the conversation to proceed to a certain point, and then whipped out his private definition of "true Scotsman" - a Scotsman that does not drink wine. The interlocutor of course had no way of knowing the speaker had a private, or special, definition of the term "true Scotsman" and so he innocently entered the conversation only to be frustrated by the speaker's stipulated definition. Lewis Carroll famously highlighted this irritating little debating trick when one of his characters grandly declared that a word meant exactly what he meant it to mean, no more and no less.
One cannot have reasonable, good-faith discussions with people that use the 'no-true-Scotsman'.