Written during World War II, The Open Society and Its Enemies was published in 1945 in London by Routledge in two volumes: "The Spell of Plato" and "The High Tide of Prophecy: Hegel, Marx, and the Aftermath".

He agrees with Schopenhauer's view that Hegel "was a flat-headed, insipid, nauseating, illiterate charlatan, who reached the pinnacle of audacity in scribbling together and dishing up the craziest mystifying nonsense."

Methodological essentialism is the view that science should aim to uncover the hidden reality or essence of things through intellectual intuition and to describe it in words through definitions.

According to Popper, Plato believed change can only lead towards imperfection and evil, which is a process of degeneration – he saw history as an illness, and set out a system of historical periods governed by a law of evolution.

Plato's ideal state was not a progressive Utopian vision of the future, but rather a historical or even pre-historical one that attempted to reconstruct ancient tribal aristocracies to avoid class war.

Plato suggests that knowledge of breeding and the Platonic Number can prevent racial degeneration, but lacking a purely rational method, it will eventually occur.

Popper also discusses Socrates' moral intellectualism and its democratic and anti-democratic aspects, as well as Plato's institutional demands regarding the highest form of education.

Popper argues that Plato's emphasis on breeding for the creation of a race of godlike men and women destined for kingship and mastery is an attempt to create a perfect society dominated by a class of rulers.

[2][11] Utopian engineering requires a blueprint of the ideal society, in rough outline at least, and determines practical action by considering the best means for realizing the ultimate political aim.

"[11][2] The piecemeal engineer aims to improve the current situation of society by gradually fighting against suffering, injustice, and war, and other such social evils whose existence can be established relatively easily.

The breakdown of the closed, tribal society, chiefly caused by the development of sea-communications and commerce, led to the partial dissolution of old ways of life, political revolutions and reactions, and the invention of critical discussion, which was free from magical obsessions.

A final addendum, added in 1965 and untitled (numbered "IV"), recommends the book Modern Dictatorship by Diana Spearman, in particular its chapter titled "The Theory of Autocracy".

Popper also discusses the conflict between the Platonic-Aristotelian philosophy and the Great Generation, and how Christianity initially opposed Platonizing Idealism and intellectualism, but changed significantly when it became powerful in the Roman empire.

He briefly discusses the development of the opinions of Fichte, who was one of the founders of German nationalism; then he goes back to criticizing "Hegel's hysterical historicism", which he views as "the fertilizer to which modern totalitarianism owes its rapid growth".

Popper argues that this interpretation is a misreading of the philosophy of Marx, who believed that economic motives were "symptoms of the corrupting influence of the social system", not "the driving forces of history".

Popper examines the effects of Marx's historical prophecy on recent European history and notes that Marxist leaders had no practical program beyond "Workers of all countries, unite!"

However, he disputes Marx's prophetic argument of the law of increasing misery and suggests that trade unions, collective bargaining, and strikes can prevent capitalists from using the industrial reserve army to keep wages down.

Popper also discusses Marx's theory of the trade cycle and surplus population, and his argument that the rate of profit tends to fall, leading to capitalism's inevitable collapse.

Chapter 21, "An Evaluation of the Prophecy", explains that Marx's prophetic conclusions drawn from the observations of contemporary economic tendencies are invalid, mainly due to the poverty of historicism as an epistemology.

Popper concludes that our views and actions are not solely determined by heredity, education, and social influences, and gives the example of Beethoven to explain the sociological aspects of his work.

Popper introduces the sociology of knowledge and argues that scientific objectivity is achieved through "the friendly-hostile co-operation of many scientists" and the public expression of criticism and opinion.

Popper proposes critical rationalism, "which recognizes the fact that the fundamental rationalist attitude results from an (at least tentative) act of faith—from faith in reason."

The choice between adopting a more or less radical form of irrationalism or critical rationalism is not just an intellectual decision but a moral one, which "will deeply affect our whole attitude towards other men, and towards the problems of social life".

Popper suggests that there is an objective way to determine if a statement corresponds to the facts and also to establish a standard in the moral and political fields, thereby countering relativism.

According to Popper, the paradox of freedom was "used first, and with success, by Plato", but was "never grasped" by Marx, who held the "naïve view that, in a classless society, state power would lose its function and 'wither away'".

"[2] As Popper wrote in academic obscurity in New Zealand during World War II, several colleagues in philosophy and the social sciences assisted with the book's path to publication.

Gombrich kept a collection of letters, which he now shares, revealing that Popper saw his book as an "urgent" new philosophy of politics and history, trying to contribute to an understanding of the totalitarian revolt against civilization.

Popper also defended the length and complexity of his book, claiming that it achieves a rare degree of lucidity and simplicity despite being "thronged with thoughts on every single page".

Gombrich's "Recollections" end on a reflective note, musing on the immense contributions that Popper made to intellectual discourse in the decades that followed.

[21] The Marxist economist Ernest Mandel identifies The Open Society and Its Enemies as part of a literature, beginning with German social democrat Eduard Bernstein, that criticizes the dialectical method Marx borrowed from Hegel as "useless", "metaphysical", or "mystifying."