In 2018, I have discovered the most harmful fallacy in the world. This logic error is worse than others, because its problem is not glaringly obvious, and has resulted in a lot of bad policy. I'm not talking about the good old standbys like ad hominem (insulting your opponent to prove him wrong), nor straw man (arguing against a caricature), even though these are very popular. It's something better (worse).

What do all these claims (made by serious people) have in common? OK, most of them are on political topics, but not that.

1. Nazis made lists of Jewish people. You made a list of Jewish people. Obviously, you're a Nazi.
2. Good software passes tests. This software passes tests. Thus it's good software.
3. Bad people have guns. You have a gun. So, you are a bad person.
4. Killing is bad. Weapons are designed to kill. Therefore, weapons are bad.
5. Discrimination can chase away women from STEM black people in mathematics. There are not many women in STEM black people in maths. Discrimination did it.

It's the same mistake over and over again: the affirming the consequent fallacy. In mathematical notation, the logic is:

if P then Q
Q
therefore P

This is wrong, terribly wrong, because there can be other causes for Q. Stopping at the first possible cause P is a cognitive shortcut - and sometimes a powerful weapon.

Let's go through each of these examples.

1. Nazis & lists. Yes, obviously Nazis are/were bad. But that was not simply because they made lists. Anyone can make lists, for all kinds of purposes. Somewhere around here, I have a list of my favorite Jewish musicians, but don't want to do away with them. But the accusation of "you're a nazi!!11!" is sufficiently toxic these days that defending oneself with elementary retorts like ... "Nazis drank water ... don't you drink water too?" can get one into trouble.

2. Software & tests. Yes, obviously it is good for software to pass tests. But it is neither strictly necessary, and definitely not sufficient. The tests may be fictional, provide poor coverage, or even contain & enshrine errors. Focusing on testing may detract from gathering actual deployment experience. Seeing good test results may produce a false confidence of actual quality. Software quality can be better measured with success of the user base & their bug reports.

3. Guns are bad, m'kay. Yes, obviously it sucks if criminals are armed. But many non-criminals also have guns, some millions of people here in Canada and perhaps a hundred million in the States. The worst thing they may do is cause accidents or suicides, but that's a tiny share. Most of them are just plain good reasonable people, and it is completely unfair to taint them with the crimes of, well, criminals. But, for a leftie politician, it is easier to punish this whole mostly-innocent class of people.

4. Another gun one, sorry: Yes, they are "designed to kill". Except they really aren't: guns are designed so that they CAN kill. They are also designed to shoot holes into paper targets. They are designed to hold their value by being robust and long-lasting. They are also designed to be able to provide some protection even if just brandishing them. They are designed to make people be ABLE to do all those things, most of them fine and legal. (Even killing is sometimes legal - for example in self-defence.) I christen this particular variety as the inverse teleological fallacy, after teleology (explaining something by reference to their design / purpose), and inverting capability into mandate. You heard it here first.

5. Yes, obviously unjustifiable discrimination is bad. But to assume that discrimination is the sole or primary cause of outcome disparities is simply wrong. There can be many other reasons, but in the present corporate/political atmosphere, even just to discuss the possibilities leads to fainting couches and firings. Worse, it leaves the other causes unexplored and thus unfixed (if they can/should be fixed at all).

The common thread is the lack of imagination to look for and quantify other causes. The danger is the tyrant's satisfaction with simplistic & weaponizable answers. Please let's try to do better, and call out others to do the same. Here's where you, cherished reader, come in. Please think of an example or two you have seen, and add a comment.