The no true Scotsman fallacy occurs when an arguer narrows a definition of a key term so as to exclude members it ordinarily would include, in order to make a sweeping generalization about that class.

Here is the kind of example that gives the fallacy its name:

Person 1: "No Scotsman adds sugar to his porridge." ​Person 2: "But my uncle is a Scotsman, and he adds sugar to his porridge." ​Person 1: "Well, no true Scotsman adds sugar to his porridge."

By narrowing the scope of what a "true" Scotsman is, Person 1 may hope to cling to a generalization about Scotsmen that isn't really true of all of them.

While in the classic case of this fallacy, it is employed after the initial generalization has been disproved by a counter-example, the fallacy can also be created in anticipation. This happens frequently in political rhetoric, with statements such as "No true conservative would ever...." or "No true liberal believes in...."

This fallacy is often an attempt to mask the fallacy of sweeping generalization.

See also: definitional retreat.