One of my interests is pseudoscience. There are a couple of lines I like to draw, between science and nonscience, and between nonscience and pseudoscience. I think science is a good thing, pseduoscience is a bad thing, and nonscience can be anything in between.
Let's start with science. Ultimately, my philosopher friend tells me, it is an impossible job to cleanly define science such that everything which is science is inside the line you draw, and nothing that isn't science is inside. Ok. So I have some fuzzy boundaries for inside vs. outside. Some things might move more cleanly inside, in time, and others will start fuzzy and then move outside. For the most part, though, the fuzziness doesn't cause me problems.
Inside science we have things where people are trying to understand the natural universe, using sharable methods and data, and is subject to further testing. Each of those elements is important. Science is about understanding. Making use of your understanding (say to build a better computer) is engineering. This is one of the many good things that isn't science. Science is about the natural universe. If you're discussing the nature of God, whether there is one, etc., you're over in theology, not science. If you, say, have an experiment where only you can get a certain result, and if I stood next to you and looked I wouldn't see what you did, then you've left science. The most difficult part, for both scientists and non-scientists, is the last -- whatever your conclusions and understanding are today, you have to be open to new data that will cause you to revise yourself tomorrow.
This last is perhaps the quickest method to find pseudoscience. The related point is, in science you don't start with your answer. The flat earth and young earth sites over in the 20 links game all start with their conclusion that the earth is flat, or young, and then assemble whatever arguments they can, however contrived and ultimately dishonest, to support their conclusion. A useful question, then, is "What evidence would cause you to change your mind?". If there's none, the person isn't interested in the science.
Having divided to science and nonscience, keeping in mind that 'nonscience' is not a slam, it's time to think about pseudoscience. I've got a shelf or two of examples on my bookcases at home. Essential to being pseudoscience is that the authors/fans/... have to claim that they are indeed doing science. Baseball, painting, theater, plumbing, ... are all good areas and aren't science. But it's also the case that none of them claim to be science. In baseball, I think the designated hitter rule is bad. This isn't a matter of science; no evidence you present will (at least nothing I've heard in 40 years on the topic) change my mind. But no problem, it isn't science.
Where we get to the pseudoscience is with the claim that it is science, even though it fails to be science. So, for instance, it is possible that the earth really is flat -- if we're being scientific we have to be open to the possibility that tomorrow we'll get a batch of fresh information about optics, gravity, etc., that will lead to the conclusion that the earth is flat after all. But the flat earthers are maintaining it in spite of the fact that there is not such evidence now.
Perhaps my favorite pseudoscience is biorhythms. It's something that could have turned out to be science. The problem only being that when it didn't, the believers didn't stop believing that it was. The idea here is that your body has certain rhythms (which it does in some things) that could predict whether you were in better or worse shape, and more or less accident prone. We're ok so far -- the topic is natural universe, the data are sharable (did people have more accidents, when were they born, where were they in their biorhythm(s), etc.). But when it came down to comparing the observations to the predictions, it failed. There were some problems with the idea (ex: how did the body maintain such a precise timing of the rhythms over a lifetime?), but if the predictions accorded with observation, that's ok. Just means more research is needed, this time to answer those questions.
I first read about this in the 1970s. At that time, one of the books mentioned that there really was a lot of data in support of the idea, but it was in a steamer trunk on a ship that sank during World War II. Bit puzzling that with almost 30 years since the sinking the proponents hadn't managed to find more data, but I was younger then and it didn't strike me as odd as it should have. In the mid-1990s, I looked again (forget the reason) and a recently published book was repeating the steamer trunk story. Come on! Another 20 years on, 50 years after the event, the fans still hadn't managed to find new data. This is 'the dog ate my homework', not science. If a process that is supposed to be going on today can't be supported with observations today, you've lost the 'sharable data' part of being science.
Pseudoscience is unfortunately common when you start looking for information about climate. On the other hand, most of it is fairly easy to identify, being not even as close to science as the biorhythm business.
1 hour ago