On Popper’s Conjectures and Refutations (1963), the first three essays.
What is science, and how is it different than pseudo-science? From philosophy? Is philosophy just pseudo-science, or proto-science, or what? Popper thinks that all legitimate inquiry is about solving real problems, and scientific theories are those that are potentially falsifiable: they make definitely predictions about the world that, if these fail to be true, would show that the theory is false.