He says that the implicitly accepted idea that truth is manifest (i.e. "we have been given eyes to see the truth, and the 'natural light' of reason to see it by", 7), at the heart of both Descartes' (rationalist) and Bacon's (empiricist) teaching, gives rise to the "curious" conspiracy theory of ignorance (see last post). For Descartes, "what we clearly and distinctly see as true must indeed be true; for otherwise God would be deceiving us". For Bacon, Nature is an open book that only an impure mind can fail to read correctly (7).
According to these lights, it is only prejudices and powers (spiritual and earthly), for example, "our sinful refusal to see manifest truth" that are to blame. Marxism seized upon this and ran with it in a certain way - it is completely unoriginal. The "priest who keeps the people in ignorance was a stock figure of the eighteenth century", and Popper believes was one of the inspirations of [classical] liberalism. Earlier yet: "protestant belief in the conspiracy of the Roman Church", and "Plato's uncle Critias" (7)
Popper says that "this curious belief in a conspiracy is the almost inevitable consequence of the optimistic belief that truth, and therefore goodness, must prevail if only truth is given a fair chance" (8). He says a tolerance based on an "optimistic faith in the victory of truth may easily be shaken", and this is liable then to turn into a conspiracy theory (which in the main was [is?] a myth as well).
He goes on: "The simple truth is that truth is often hard to come by, and that once found may be easily lost again" (8 - he speaks of erroneous beliefs with great staying power in science and medicine in particular). And yet, he says
"perhaps the strangest thing in this story is that this false epistemology was the major inspiration of an intellectual and moral revolution without parallel in history. It encouraged men to think for themselves. It gave them hope that through knowledge they might free themselves and others from servitude and misery. It made modern science possible. It became the basis of the fight against censorship and the suppression of free thought. It became the basis of the nonconformist conscience, of individualism, and of a new sense of man's dignity; of a demand for universal education, and of a new dream of a free society. It made men feel responsible for themselves and for others, and eager to improve not only their own condition but also that of their fellow men. It is a case of a bad idea inspiring many good ones" (8, italics mine)But, he says, this idea also led to disastrous consequences as this theory is "the basis of almost every kind of fanaticism" (including the idea that those who don't "see manifest truth must be possessed by the devil"). "Only those who have every reason to fear truth can deny it, and conspire to suppress it."
Popper summarizes at this point by saying that "since "truth is not manifest, as a rule", an optimistic epistemology then also leads to authoritarianism, although "perhaps less directly than does a pessimistic epistemology". "The allegedly manifest truth is therefore in constant need not only of interpretation and affirmation, but also of re-interpretation and re-affirmation", and the authorities required to do this may "learn to do so arbitrarily and cynically". Finally, "many disappointed epistemologists will turn away from their own former optimism and erect a resplendent authoritarian theory on the basis of a pessimistic epistemology (Plato)" (8,9)
My summary: - Popper teaches as he does because he - through good parenting, hard work, education, and knowledge of the ways of scientific discovery (he didn't say this [yet at least!]) - is able to see that it is manifestly true that it is not manifestly true that truth is manifest.