Popper's falsificationist strategy of conjectures and refutations can only deliver negative knowledge. It shows certain scientific theories are false, but it never shows that any theory is true.

i don't understand why he calls this a flaw. refuting positivism was one of the main purposes of popper's argument (as i understand it, anyway). in both of the counterexamples he brings up-- that smoking cigarettes causes lung cancer and that matter is made of atoms-- neither needs to be strictly capital-t True in order for us to be able to apply those theories to make useful predictions about the world. in fact neither statement is capital-t True in the sense accepted by most skeptics. plenty of people smoke cigarettes and never develop lung cancer, and no one has directly observed an atom via their senses. Popperian falsificationism has a lot of implications about the nature of knowledge revealed by scientific methods but i don't necessarily think those implications are so disagreeable as to render his argument unusable. in fact i think we might be better off if this kind of understanding of the nature of scientific theories were more widespread.