No true Scotsman
No true Scotsman, or appeal to purity, is an informal fallacy in which one attempts to protect their universal generalization from a falsifying counterexample by excluding the counterexample improperly. Rather than abandoning the falsified universal generalization or providing evidence that would disqualify the falsifying counterexample, a slightly modified generalization is constructed ad-hoc to definitionally exclude the undesirable specific case and counterexamples like it by appeal to rhetoric. This rhetoric takes the form of emotionally charged but nonsubstantive purity platitudes such as "true, pure, genuine, authentic, real", etc.
Person A: "No Scotsman puts sugar on his porridge."
Person B: "But my uncle Angus is a Scotsman and he puts sugar on his porridge."
Person A: "But no true Scotsman puts sugar on his porridge."
- not publicly retreating from the initial, falsified assertion
- offering a modified assertion that definitionally excludes a targeted unwanted counterexample
- using rhetoric to hide the modification
An appeal to purity is commonly associated with protecting a preferred group. Scottish national pride may be at stake if someone regularly considered to be Scottish commits a heinous crime. To protect people of Scottish heritage from a possible accusation of guilt by association, one may use this fallacy to deny that the group is associated with this undesirable member or action. "No true Scotsman would do something so undesirable"; i.e. the people who would do such a thing are tautologically (definitionally) excluded from being part of our group such that they cannot serve as a counter-example to the group's good nature. 
Origin and literature
The description of the fallacy in this form is attributed to British philosopher Antony Flew, because the term originally appeared in Flew's 1971 book An Introduction to Western Philosophy. In his 1975 book Thinking About Thinking, he wrote:
Imagine some Scottish chauvinist settled down one Sunday morning with his customary copy of The News of the World. He reads the story under the headline, 'Sidcup Sex Maniac Strikes Again'. Our reader is, as he confidently expected, agreeably shocked: 'No Scot would do such a thing!' Yet the very next Sunday he finds in that same favourite source a report of the even more scandalous on-goings of Mr Angus McSporran in Aberdeen. This clearly constitutes a counter example, which definitively falsifies the universal proposition originally put forward. ('Falsifies' here is, of course, simply the opposite of 'verifies'; and it therefore means 'shows to be false'.) Allowing that this is indeed such a counter example, he ought to withdraw; retreating perhaps to a rather weaker claim about most or some. But even an imaginary Scot is, like the rest of us, human; and none of us always does what we ought to do. So what he is in fact saying is: 'No true Scotsman would do such a thing!'
In his 1966 book God & Philosophy, Flew described the "No-true-Scotsman Move":
In this ungracious move a brash generalization, such as No Scotsmen put sugar on their porridge, when faced with falsifying facts, is transformed while you wait into an impotent tautology: if ostensible Scotsmen put sugar on their porridge, then this is by itself sufficient to prove them not true Scotsmen.— Antony Flew
The essayist David P. Goldman, writing under his pseudonym "Spengler," compared distinguishing between "mature" democracies, which never start wars, and "emerging democracies", which may start them, with the "no true Scotsman" fallacy. Spengler alleges that political scientists have attempted to save the "US academic dogma" that democracies never start wars against other democracies from counterexamples by declaring any democracy which does indeed start a war against another democracy to be flawed, thus maintaining that no true and mature democracy starts a war against a fellow democracy.
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Robert Anderson argues that the phrase "no true Scotsman" is not always fallacious: it depends on the syntactical context of the term "true" inserted into the phrase "no Scotsman".
- No True Scotsman, Internet Encyclopedia of Philosophy
- Curtis, Gary N. "Redefinition". Fallacy Files. Retrieved 2016-11-12.
- Antony Flew, God & Philosophy, p. 104, Hutchinson, 1966.
- Antony Flew (1975). Thinking About Thinking (or, Do I Sincerely Want to be Right?). Fontana/Collins. p. 47.
- Govier, Trudy. "A Practical Study of Argument: Looking At Language: Persuasive Definitions". SemanticsScholar. Retrieved 2021-01-25.
- Goldman, David P. (31 Jan 2006). "No true Scotsman starts a war". Asia Times. Archived from the original on 5 January 2019. Retrieved 1 December 2014.
political-science professors... Jack Mansfield and Ed Snyder distinguish between "mature democracies", which never, never start wars ("hardly ever", as the captain of the Pinafore sang), and "emerging democracies", which start them all the time, in fact far more frequently than do dictatorshipsCS1 maint: unfit URL (link)
- Robert Ian Anderson, "Is Flew’s No True Scotsman Fallacy a True Fallacy? A Contextual Analysis", P. Brézillon et al. (Eds.): CONTEXT 2017, LNAI 10257, pp. 243–253, 2017. doi:10.1007/978-3-319-57837-8_19
- "Obituary: Prof. Antony Flew", The Scotsman, 16 April 2010