Reformulation of popper
Popper starts from Hume to address the problem of induction. He agrees with him for what concerns the logical problem: we have no logical reason whatsoever to adopt induction.
Nevertheless, from the psychological point of view, Hume bases his argument on custom, so on the repetition of events. Even if Hume himself acknowledges that this doesn’t work in every case, he believes that we form expectations as a matter of habit.
Popper dislikes this interpretation, since he points out that we form expectations even after a single occurrence of an event; therefore, expectations are based of what’s interpreted by us.
Before any repetition, there must always be a point of view, and such point of view doesn’t make logically and objectively clear which repetition actually occur.
From this derives that we interpret events independently from their repetition, there are elements related to the individual which link instances and form expectations.
Hume’s point of view is passive: repetitions gradually strengthen our expectations, which are passively developed; to Popper, instead, we actively impose regularities upon our observations, we naturally tend to expect something we haven’t proved yet.