It gets its name from an almost certainly apocryphal story, in which a serial rapist and killer is being pursued by the police in Glasgow, and a Scottish MP encourages the police to search amongst the immigrant population of the city. "No Scotsman would do such a thing," the MP said.
When the criminal was caught, and turned out to be 100% Scottish, the MP was challenged about his remark.
"Well," he said, drawing himself up, "no true Scotsman would have done such a thing!"
The crux of this fallacy is that if you make a statement that turns out, in view of evidence, to be false, all you do is shift your ground -- redefine the terms so as to make your original point unassailable.
Very few other fallacies have such a capacity for making me want to smack my forehead into a wall as this one. Someone who commits this fallacy can't be pinned down, can't be backed into a corner, can't receive his comeuppance from the most reasoned argument, the most solidly incontrovertible evidence. The dancing skills of a master of the No True Scotsman fallacy are Dancing With The Stars quality.
All of this comes up because of an online discussion that I read, and (yes) participated in, a couple of days ago, on the topic of the demonstrability of evolution. Someone, ostensibly a supporter of evolution but seemingly not terribly well-read on the subject, was using such evidence as the fossil record as a support for the idea. A creationist responded, "The fossil record, and fossil dating, are inaccurate. You evolutionists always think that bringing us a bunch of bones and shells proves your point, but it doesn't, because no one can really prove how old they were, and none of them show one species turning into another. You can't show a single example, from the present, of one species becoming another, and yet you want us to believe in your discredited theory."
Of course, I couldn't let a comment like that just sit there, so I responded, "Well, actually, yes, I can. I know about a dozen examples of speciation (one species becoming another) occurring within a human lifetime."
Challenged to produce examples, I gave a few, including the ones that I described in an earlier post (Grass, gulls, mosquitoes, and mice, February 9, 2012), and then sat back on my haunches with a satisfied snort, thinking, "Ha. That sure showed him."
Well. I should have known better. His response, which I quote verbatim: "All you did was show that one grass can become another grass, or a mosquito can become another mosquito. If you could show me a mosquito that turned into a bird, or something, I might believe you."
Now, hang on a moment, here. You asked me for one thing -- to show one species turning into a different species, in the period of a few decades. I did so, adhering to the canonical definition of the word species. And now you're saying that wasn't what you wanted after all -- you want me to show one phylum turning into a different one, in one generation?
I sat there, sputtering and swearing, and not sure how to answer. So I said something to the effect that he'd pulled a No True Scotsman on me, and had changed the terms of the question once he saw I could answer it, and he'd damned well better play fair. He humphed back at me that we evolutionists couldn't really support our points, and we both left the discussion as I suspect most people leave discussions on the internet -- unconvinced and frustrated. So I was pondering the whole thing, and after taking my blood pressure medications I had a sudden realization of where the confusion was coming from. It was from the idea of a type of organism.
Most people who aren't educated in the biological sciences (and I'm not including just formal education, here; there are many people who have never taken a single biology class and know plenty about the subject) really don't understand the concept of species. They think in types. A bird is one type of thing; a bug is a different one. If you pressed them, they might admit that there were a few types of birds that seemed inherently different. You have your big birds (ostriches), your medium-sized birds (robins), and your little birds (hummingbirds). I've had students that have thought this way, and when they hear I'm a birdwatcher, they seem incredulous that this could be a lifelong avocation. Wouldn't I run out of new birds to see pretty quickly? When I tell them that there are over 10,000 unique species of birds, they seem not so much awed as uncomprehending.
The phylogenetic tree of birds (Class Aves) [credit: Dr. Gavin Thomas, University of Sheffield, UK]
I suspect that the source of this misapprehension is the same as the source of the general misapprehension regarding the antiquity of the Earth and the origins of life: the Bible. In Leviticus 11, where they go through the whole unclean-foods thing that eventually would be codified as the Kosher Law, they split up the natural world in only the broadest-brush terms; you have your animals that have hooves and chew the cud, various combinations of ones that don't, creatures that have fins and scales and ones that don't, insects that jump and ones that don't, and a few different classes of birds (which, to my eternal amusement, includes bats). And that's pretty much it. Plants were sorted out into ones that had edible parts (wheat, figs, olives), ones that had useful wood (boxwood, cedar, acacia), and ones that had neither of the above (thorn bushes). And these distinctions worked perfectly well for a Bronze-Age society. It kept you from eating stuff that was bad for you, told you what you could build stuff from, and so on. But as a scientific concept, the idea of "types of living things" kind of sucks. And yet it still seems to live on in people's minds, lo unto this very day.
So, anyway, that was my brief excursion into that least useful of endeavors, the Online Argument. It gave me a nice example of the No True Scotsman fallacy to write about here. And it really didn't affect my blood pressure all that much, but it did make me roll my eyes. Which seems to happen frequently when I get into conversations with creationists.