The fact that "pics or it didn't happen" is equivalent to calling someone a liar is an important point, because it illustrates why some of these fallacies are easy to fall into -- because there are some times when they aren't fallacies. E.g., if you know that someone has lied to you before, repeatedly, then "show me proof or I won't believe you" is The Boy Who Cried Wolf. It's not a bad idea to mistrust someone whom you know to be a liar: in legalese, the term is falsus in uno, falsus in omnibus: "false in one thing, false in everything". This isn't universally applicable: someone who might lie to cover up an affair might NOT lie about a crime they'd witnessed, for example. But in most jurisdictions in the U.S., to the best of my knowledge, someone who has previously been convicted of perjury -- i.e., they lied under oath and the prosecution was able to prove beyond a reasonable doubt that they lied under oath -- will not be allowed to testify about something else. So while it's technically a fallacy to say "proof or I won't believe you" to someone who's lied to you before -- because you have no proof that they're lying this time -- it's not a bad idea, because that's the way to bet.
But... when you apply that to someone whom you have no reason to suspect of lying, then it's the Argument From Silence fallacy. And, as Ryoshun Higoka has pointed out, it's also really rude to be calling someone a liar based on no evidence at all. If you find their claim hard to believe, that's one thing -- "You're telling me that FFG already sent you a copy of the Core Set, months ahead of the official release date? Pics or it didn't happen!" There, there's some evidence to suggest that they are lying about this claim. But when you know of no evidence to suggest that the person is lying, then it's rather rude (to say the least) to blithely assume bad faith.
There's also a point I want to mention about No True Scotsman, which comes in two forms. One of them is a fallacy and one of them isn't. Remember, a fallacy is invalid reasoning in a logical argument. Now let's look at two similar statements:
Example 1: No Samurai would ride cavalry. (Friend: "But the Unicorn Samurai ride cavalry all the time.") Okay, well, no true Samurai would ride cavalry.
This is an attempt to defend the earlier statement by redefining the terms; a classic example of the No True Scotsman fallacy. But this one...
Example 2: No Samurai would ride cavalry. (Friend: "But the Unicorn Samurai ride cavalry all the time.") Which is why although they claim to be Samurai, I refuse to consider them true Samurai.
This isn't a good, well-laid out logical argument (the guy certainly hasn't proven his initial statement), so I often see people claiming that this is also the No True Scotsman fallacy. But in fact, the guy is making a different claim. He is claiming that there is a certain category he calls "true" Samurai, with a certain standard of behavior that one must measure up to. Otherwise, you're in the other category, "Samurai in name only". You may call yourself a Samurai, but that doesn't in fact make you a Samurai, because you have failed to measure up to the standard of behavior. Just as living in an airport hanger and calling yourself an airplane doesn't make you an airplane in truth, because you fail to measure up to the "can fly when given jet fuel" standard.
Now, this second form can still be a flawed argument, because it might contain a false premise. If nobody else agrees with the standard of behavior the guy is laying out, they will say "You're talking nonsense. Riding cavalry has nothing to do with one's birth. The Unicorn Samurai are still Samurai, because they were born into the Samurai class." But his argument might contain a true premise. Maybe it's a commonly-accepted standard in Rokugan that Bushi should do their fighting on foot, and using a horse is considered to be "cheating" in some way. The Unicorn Bushi, although born into the Samurai class, are thus breaking the accepted social rules, and may find themselves shunned by those who consider themselves to be the "true" Samurai, because they're acting in ways that are "beneath their station". (See also 18th and 19th century British society, as portrayed in LOTS of literature, for more examples of how social class was governed by somewhat rigid rules, and how breaking those rules would find you dishonored because "No "true" gentleman would act that way.")
So my first example is indeed an illustration of the No True Scotsman fallacy. But the second one, although having the same form of words as the No True Scotsman fallacy, is in fact something quite different: a statement that there are certain rules of behavior to belonging in a certain class, and that person X is not living up to those rules of behavior. And thus, although X claims to belong to class Y, since they don't measure up to the rules of class Y, their claim is false. This is not a fallacy. It can be an incorrect argument if its premise is false (the rules of class Y are not as rigid as the claimant is claiming that they are), but it is a valid argument that follows logically from its premises. And it is often mistaken for the No True Scotsman fallacy.
I think the reason I care enough about this to have spent an hour writing up this post is because I've seen too many cases of "That's a fallacy" used, well, fallaciously. Especially in the case of the No True Scotsman argument, because so few people recognize that it is not always a fallacy. E.g., "No true member of the Honors Society would cheat on a test. You have been caught cheating, and you are therefore expelled from the Honors Society. You may continue to claim to be a member if you want -- we can't stop you from saying whatever untrue things you want to say -- but you will not be a true member, and you will be lying if you claim to be one." That's not a fallacy. But I've seen people claim that that was the No True Scotsman fallacy, and refuse to listen to any reasoning otherwise. It was extremely frustrating. It's very important to be able to recognize fallacies, including things that look like fallacies but aren't, and it can take a lot of practice. It's worth doing, though, because the more you can learn to recognize actual good reasoning and tell it apart from fallacies, the less likely it is that you'll be fooled into believing things that aren't, in fact, true.
Edited by rmunn
