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On Popper’s view, the growth of knowledge begins with a problem, which is usually an inconsistency discovered either within an inherited theory, or between inherited theories, or between an inherited theory and an accepted observation statement. We attempt to solve the problem by proposing new explanatory theories.
We criticise these theories in various ways and we evaluate them as better or worse solutions to our problem. As a result, we usually come to understand the problem better, which leads us to suggest further new theories, which are in turn criticised and evaluated. It may be that we eventually settle on one theory as clearly better than its available rivals (as happened in the case of Newton’s theory in the eighteenth century). That dominant theory may then become a main focus for our criticism, identification of new problems and further development of theory. The attempt to resolve the problems that the criticism of the dominant theory generates may lead to new conjectures which are rivals to that theory and which manage to stand up to criticism better than that theory does, thus leading to its revolutionary overthrow.
The growth of knowledge is thus a process of conjecture and criticism. An important kind of criticism is empirical, that is, the discovery of points at which a conjecture clashes with reality as we experience it. Popper proposes that what makes a conjecture scientific is its susceptibility to empirical criticism, that is, its falsifiability. A conjecture incapable of such an empirical clash is deemed metaphysical. However, falsifiability may be either direct or indirect; and that enables some conjectures, which are metaphysical in that they are not directly falsifiable, to qualify as scientific because they are indirectly falsifiable. The process of conjecture and criticism leads to progress only when criticisms are not evaded in pseudo-scientific ways. What distinguishes science is not just that its theories are, directly or indirectly, falsifiable but also that its procedures are not ad hoc. That requires that an amendment to a theory that removes an inconsistency identified by criticism is acceptable only if it solves some problem in addition to the problem of removing the inconsistency.”
Danny Frederick, ‘Against the Philosophical Tide’.

“An idea which is at one time untestable, and therefore metaphysical, may with changed circumstances become testable and therefore scientific. (…) Not only can a methaphysical theory be meaningful, it may actually be true; but if we have no way of testing it there can be no empirical evidence for it, and therefore it cannot be held to be scientific. Even so, theories which cannot be empirically tested may still be critically discussed, and have the arguments for and against them compared, as a result of which one of them may appear preferable to another.”
Bryan Magee, 'Popper

For Popper, what neopositivists do: To calculate the probability that the results of an investigation agree with the hypotheses is still "justificationism" - he also calls it "verification" or "corroboration", although he likes this term more for his own logic research -. Calculating probabilities is verifying based on prior knowledge of what is already known. For Popper, true falsifiability, and therefore, the true way of doing science, is that one in which the probabilities (according to the same neopositivist logic) that the hypothesis is false are far much higher than that it is true. He is firm and clear in that regard from the beginning.

Dragon in my garage As an example of skeptical thinking, Sagan offers a story concerning a fire-breathing dragon that lives in his garage. When he persuades a rational, open-minded visitor to meet the dragon, the visitor remarks that they are unable to see the creature. Sagan replies that he "neglected to mention that she's an invisible dragon". The visitor suggests spreading flour on the floor so that the creature's footprints might be seen, which Sagan says is a good idea, "but this dragon floats in the air". When the visitor considers using an infra-red camera to view the creature's invisible fire, Sagan explains that the fire is heatless. He continues to counter every proposed physical test with a reason why the test will not work.

Sagan concludes by asking: "Now what's the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all? If there's no way to disprove my contention, no conceivable experiment that would count against it, what does it mean to say that my dragon exists? Your inability to invalidate my hypothesis is not at all the same thing as proving it true."