No true Scotsman, or the self-sealing fallacy, is an ad hoc style fallacy of equivocation and question begging. It was advanced by philosopher Antony Flew in his 1975 book Thinking About Thinking – or do I sincerely want to be right?.
In putting forward the above rebuttal one is equivocating in an ad hoc attempt to retain an unreasoned assertion. The proposer initially treats the definition of "Scotsman" (i.e, a man of Scottish ancestry and connection) as fixed, and says that there exists no predicated case that falls within that definition. When one such case is found, the proposer shifts to treat the case as fixed, and rather treats the boundary as debatable. The proposer could therefore be seen prejudicially not to desire an exact agreement on either the scope of the definition or the position of the case, but solely to keep the definition and case separate. One reason to do this would be to avoid giving the positive connotations of the definition ("Scotsman") to the negative case ("sex offender") or vice versa.
The truth of a proposition depends on its adequacy to its object ("Is the drawing a true likeness of Antony Flew?"). The truth of an object depends on 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, for example when it is vague or contested.
"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", and the corresponding feeling that the concepts 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 a short step from that feeling to treating one's own definition, however arbitrary, 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.