Pam this is the very definition of the No True Scotsman logical fallacy.
No true Scotsman is a term coined by
Antony Flew in his 1975 book
Thinking About Thinking. It refers to an argument which takes this form: Argument: "No Scotsman puts sugar on his porridge."Reply: "But my uncle Angus likes sugar with his porridge."Rebuttal: "Ah yes, but no
true Scotsman puts sugar on his porridge." This form of argument is a
fallacy if the predicate ("putting sugar on porridge") is not actually contradictory for the
accepted definition of the subject ("Scotsman"), or if the definition of the subject is silently adjusted after the fact to make the rebuttal work.
The truth of a proposition is its adequacy to its object ("Is the drawing a true likeness of Antony Flew?"). The truth of an object is its adequacy to its concept ("Is the figure drawn on the paper a true triangle?"). Problems arise when the definition of the concept has no generally accepted form. "A true Scotsman" (a concept) is not on the same level as "a true triangle" (a concept) never mind "the true Antony Flew" (a concrete existing object). The formal similarity, "true X", the feeling that they should be on the same level, in some sense must be on the same level (even perhaps all exist as objects), motivates the fallacy. It is short step from that feeling to treating one's own definition of a "true Scotsman" (who else's?) as having the same objectivity as that of a geometrical figure or an existing individual, and then attempting to make the world agree.
Some elements or actions
are exclusively contradictory to the subject, and therefore aren't fallacies. The statement "No
true vegetarian would eat a beef steak" is not fallacious because it follows from the accepted definition of "vegetarian": Eating meat, by definition, disqualifies a (present-tense) categorization among vegetarians, and the further
value judgement between a "true vegetarian" and the implied "false vegetarian" cannot likewise be categorized as a fallacy, given the clear disjunction.
Using the context of culture, individuals of any particular religion, for example, may tend to employ this fallacy. The statement "no true
Christian" would do some such thing is often a fallacy, since the term "Christian" is used by a wide and disparate variety of people. This broad nature of the category is such that its use has very little meaning when it comes to defining a narrow property or behaviour. If there is no
one accepted definition of the subject, then the definition must be understood in context, or defined in the initial argument for the discussion at hand.
It is also a common fallacy in politics, in which critics may condemn their colleagues as not being "true"
Communists,
liberals or
conservatives because they occasionally disagree on certain matters of policy. It comes in many other forms - "No decent person would" - it is argued "support hanging/watch pornography/smoke in public", etc. Often the speaker seems unaware that he/she is, in fact, coercively (re)defining, 'objectifying', what the phrase "decent person" means to include/exclude what he/she wants and NOT simply following what the phrase is already accepted as meaning. The argument shifts the debate from being about hanging/pornography/smoking and tries to make it seem that anyone disagreeing with the speaker is arguing for the "indecent".
Allow me to demonstrate..
"No Christian will fall away"
"I know a Christian that fell away"
"Oh, well no true Christain will fall away!"
This is a logical fallacy. It is like asserting that 2+2=9