|Karl Popper, possibly in need of some simple English.|
Lots of people think that what makes something count as science is the fact that it came from observation and testing. But I don't buy that. Plenty of stuff that doesn't count as science is all about observation. People believe in astrology, for instance, because they observe that astrologers make predictions that turn out to be true. So why isn't astrology science? How is the theory of astrology different from, say, Einstein's theory of general relativity?
The difference is that Einstein's theory might turn out to be wrong, and if it is, we'll eventually know. We'll know because one day we'll make observations about the world that aren't in line with his theory. What makes theories like Astrology, Freudian analysis, and other sorts of pseudo-science unscientific is that they can explain everything. Usually, when we see that a theory is confirmed over and over again, we believe in it even more. But if there's no way at all, even in principle, to make an observation that isn't in line with the theory, then all those confirmations don't actually mean anything. Theories like that would be in line with all the same observations even if the theories were false--so if the theory is false, there's no way to find that out.
General relativity, evolution, Newtonian mechanics, and Mendelian genetics are all scientific theories not because there's lots of evidence confirming them, but because they make falsifiable predictions. They predict certain things about the world, and the predictions are risky because we can check to see if the world really is that way. If the world doesn't turn out to be the way the theory predicts, then we know the theory is false. For pseudo-science, we get all the same predictions whether the theory is true or not. There's no observation we could make to find out whether the theory's false. Unscientific theories are unfalsifiable, unable to be shown false.
Observations that support a theory only really count as support if the theory makes risky predictions. If a theory is scientific, you should be able to make a test so that if you get one result, you can continue believing the theory just as much as you did before--but if you get another result, you have to conclude that the theory is false. Pseudoscience doesn't let you make these kinds of tests, because there's never any result you could possibly get that would make you change your mind and stop believing the theory.
Sometimes people have theories that really are testable, but when the test results don't come out the way they want, they either find some excuse to throw those results away, or they change their theory to match the results so it looks like they were right all along. That's not science either, because it's impossible to find out that the theory is false when you do things that way, too.
This philosophy of science is called falsificationism, and I made it because draws it a line between what is science and what isn't.