6 ideas
Asking 'who should rule?' invites the answer 'the best' and leaves no safeguard when rulers turn out bad. Popper replaces it with 'how can we organize institutions so that bad rulers can be removed without bloodshed?' He defines democracy by this capacity for peaceful dismissal, not by the principle of majority rule.
Utopian engineering sets out an ideal end-state and rebuilds society wholesale to reach it. Because the change is so large, no one can trace which action caused which effect, so the plan cannot learn from its errors and ends by suppressing criticism. Piecemeal engineering makes small, reversible reforms that can be tested. It aims to remove concrete, identifiable suffering rather than to realize an abstract ideal good.
Historicism holds that history unfolds by discoverable laws of development, so the thinker's job is to predict the destined future and help it along. Popper argues that no such unconditional laws of history exist. Treating one's program as history's verdict excuses moral responsibility and justifies sacrificing present people to an imagined endpoint.
A closed society is tribal and organic. Its customs are experienced as unchangeable, like natural laws, and individuals have no standing to question them. An open society recognizes that norms are human-made and revisable, so individuals must take personal responsibility for decisions. The strain of carrying that responsibility produces a recurring nostalgia for the security of the closed, tribal order.
Unlimited tolerance extended to the intolerant lets them destroy the tolerant society and tolerance with it. Popper holds that intolerant views should be countered by argument while possible. A society may still claim the right to suppress movements that reject rational discussion and answer argument with violence.
Read against the grain, Plato's ideal state is a program to arrest social change. It does this through rigid caste hierarchy, censorship, the 'noble lie', and rule by a self-appointed philosopher class. Plato recast justice as 'everyone keeping to their class', converting a moral concept into a tool of collective stability over individual rights.