While Popper was primarily interested in the philosophy of science in this book, I believe his insights apply to all of epistemology, not just the study of the material world. As a classical liberal, Popper extends his epistemic reasoning out to his own version of social contract theory. I think that, while he had a good basis to work off of and an amazing intellect, he made the mistake that many classical liberals made: he forgot that the institutions he advocated for would never go away; where tolerance, as he imagined it, was only supposed to be implemented so long as it was practically useful to collective flourishing, it has become the monster that it is today... inspired by his own words.
So, please read Conjectures and Refutations. It will help broaden your understanding of how one can say that they know what they know, how science as an exercise ought to be done, and reveal a great deal of the social philosophy that has gotten the western world into the trouble that it is in now.