Popper, brought up in Vienna, was well aware of the Vienna Circle. That school of logical positivism, led by Moritz Schlick, defined knowledge (especially scientific knowledge) as being propositions which could be verified. Popper thought this was quite wrong. In his opinion, science grew indirectly, by wrong ideas being "falsified". This he worked up in great detail in a series of books, of which "The logic of scientific discovery" is the most famous. All philosophy of science since then has had to deal with this issue, the "criterion". By 'criterion' is meant: what is it that makes a theory truly scientific, as opposed to simply common sense or opinion?