After reading through this discussion, I think you have a somewhat better understanding of Popper.
@Ren did a really good job, but there are a few things I want to add.
According to the criterion of demarcation, we can never be 100 percent certain that our theories are "true". Although truth is most certainly possible, certain and absolute truth are not. To understand what this means, we need to explore Poppers philosophy a little. His entire epistemology stems from one basic assumption: that all humans are fallible. That we are all prone to error, and that therefore we can be certain of nothing. In the pursuit of truth, our fallibility opens us up to mistakes. Because we are fallible, we are going to make mistakes, and there is nothing we can do about it. But if we are flawed, how is truth possible? How is it that we can even begin to distinguish truth from fiction? This, roughly speaking, is the problem Poppers epistemology tries to solve.
Popper explains the only way to overcome our fallibility is through the process of "conjecture and criticism", of "trial and error". Whenever we observe an anomaly that has no explanation, what we will naturally do is guess. We will make a bold conjecture about what we think the explanation might be. If the power suddenly goes out in your home, you might guess that the power is down on the entire street. You might test your conjecture by going outside and looking at your neighbors houses. If its dark, and you see no lights, you might feel comfortable with your conclusion. But you could be wrong. There might be an alternative explanation why all the lights are out. It might just be a freak coincidence. Indeed, the real explanation why your power went out might be that your power company has simply cut you off because of an overdue bill. But those are not your only options, there are more. The circuits in your house might be damaged, you may have blown a fuse, an EMP could have blown all the electricals in your house. Admittedly, the last three possibilities are not very realistic, but this is not important. What's important here is that you see there is always more than one possible explanation; one possible conjecture. With every anomaly, there are always multiple competing explanations. Your goal, as a rational agent, is to find a way of ruling them out.
This is where criticism becomes important. In the example above, you may have walked outside to prove your hypothesis. You may think that the observation "all the lights on my street are out" is a piece of confirming evidence that aught to make you more confident in your conjecture. As if hypotheses are confirmed by piling evidence for or against competing options. This attitude, Popper claims, is irrational. We won't get into the reasons here, however, he says that the true purpose of evidence is to rule out competing explanations. This idea that we use it to "confirm" hypotheses is a misconception. When you see that all the lights on your street are out, it is more rational to consider it as a refutation of other hypotheses; that it rules out the possibility that your power company cut you off, or that you blew a fuse somewhere. Well actually, if you're anything like me, this claim should make you feel a little uneasy. If you feel that this argument is too simple or too clean, then you should trust your instincts. Because as I've presented his argument here, it IS too simple and too clean. Surely, you might be thinking, the observation that "all the lights on the street are out" is not enough to rule out the possibility that your power has been cut off. Surely claiming that we have successfully narrowed down our options to only one tenable conjecture is itself irrational. If indeed this is what you are thinking, then you are correct. Gold star!
In reality, the only way to narrow down your options is through argument. Not through evidence, not through induction, not through intuition or instinct, but rather through argument. Only argument ever justifies anything, and only argument can ever be used to rule out competing explanations. However, since all humans are fallible and prone to error, even these refutations must be accepted only tentatively. Not only can we never be sure of anything we know, we can also never be sure of our criticisms and refutations. Above I explained that the only way to overcome fallibility is through conjecture and criticism. The reason is that the process of narrowing down your options helps you to overcome error. Whenever you look for evidence, you are looking for ways or "eliminating" mistaken conjectures. For ways of removing irrational hypotheses. Indeed, all of your efforts may be mistaken. You might not just be mistaken in your hypotheses, but also in what you believe is necessary to rule them out. And this is where the beauty of science steps in. One individual might not have much of a chance discovering truth. But a whole community of rational minded individuals with the single minded pursuit of discovering true theories, we have a much better chance of eventually getting there. Better to use a whole community of super computers rather than just one.
