Imagine Hamish McDonald, a Scotsman, sitting down with his Glasgow Morning Herald and seeing an article about how the "Brighton Sex Maniac Strikes Again." Hamish is shocked and declares that "No Scotsman would do such a thing." The next day he sits down to read his Glasgow Morning Herald again and this time finds an article about an Aberdeen man whose brutal actions make the Brighton sex maniac seem almost gentlemanly. This fact shows that Hamish was wrong in his opinion but is he going to admit this? Not likely. This time he says, "No true Scotsman would do such a thing."
—Antony Flew, Thinking about Thinking (1975)
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.