"ISIS are not true Islam."

 

"You were never a true Christian."

 

This is a dishonest technique where the user intends to exclude from their definition anything which may go against their definition. So in the first example above, if someone wants to argue that Islam is a religion of peace, they can simply say that anyone who is violent is not a "true Muslim". In the second example, often aimed at atheists who are ex-Christians, they try to maintain the idea that no one leaves Christianity by excluding from the definition anyone who does leave. This is meaningless; "true Christian/Muslim" doesn't have a proper definition.

 

The name of the fallacy comes from the following example of its use:

 

Me: "No Scotsman would commit a rape."

You: "I can tell you several Scotsmen who have commited rape."

Me: "No true Scotsman would commit a rape."

 

It is a fallacy because the phrase "true [group member]" does not mean anything, unless you properly define it first. What is a true Scotsman? If it doesn't simply mean a Scotsman, then you are referring to some unspecified subset of Scotsmen. And if you don't define exactly who you are talking about, you can make that group fit whatever definition you like. The problem is that this group is an arbitrary invention, so doesn't mean anything. In the same way, if a "true Christian" is anything other than a Christian, then it's a meaningless phrase unless carefully defined.