A systematic exposition of Popper’s philosophy of science, fallibilism, and the open society
Introduction: The Problem of Induction and the Demarcation Criterion
We begin at the opening of Popper’s argument, where he establishes the foundational problem that drives his entire philosophical project: the problem of induction and the need to distinguish science from non-science.
Popper begins by identifying the traditional view of scientific method, which he traces from Francis Bacon through the logical positivists of the Vienna Circle. The standard account holds that science proceeds by observing facts, generalizing from them through induction, and then confirming those generalizations by further observation. Popper shows that this picture is flawed at its core. The logical problem of induction, first formulated by David Hume, demonstrates that no number of singular observation statements can logically entail a universal statement. No matter how many white swans we observe, we cannot deduce that *all* swans are white. Induction cannot be justified by logic, and attempts to justify it by appeal to experience merely shift the problem, since those appeals themselves rely on inductive reasoning.
But Popper does not stop at diagnosing the problem. He offers a radical solution: abandon induction altogether. “The empirical basis of science is not observation statements but rather the critical method itself.” The crucial move is to recognize that science does not need a method for arriving at true theories; it needs a method for eliminating false ones. This leads directly to his demarcation criterion: a theory is scientific if and only if it is falsifiable — that is, if it makes predictions that could, in principle, be shown false by observation. Astrology, Marxism in its unfalsifiable forms, and psychoanalysis are not scientific not because they are false, but because they cannot be tested. They accommodate any possible observation by adding ad hoc hypotheses. Genuine science, by contrast, puts itself at risk: it specifies in advance what would count as a refutation.
The significance of this opening move cannot be overstated. Popper has redefined the entire task of philosophy of science. Where earlier thinkers sought to justify scientific knowledge by grounding it in secure foundations, Popper argues that no such foundations exist. Science progresses not by accumulating verified truths but by eliminating errors through conjectures and refutations. The scientist does not begin with observations but with bold guesses — conjectures — which are then subjected to the most severe tests possible. Only those conjectures that survive testing are retained, and only provisionally.
“The criterion of the scientific status of a theory is its falsifiability, or refutability, or testability.”
Part One: The Logic of Scientific Discovery
We move into the first major division of the work, where Popper develops the complete logical structure of his critical rationalism as applied to the natural sciences. This section systematically builds the replacement for inductive methodology.
Popper begins Part One by distinguishing between psychology of knowledge and logic of knowledge. He is not concerned with how scientists actually arrive at their ideas — that is a matter for empirical psychology and history. His focus is exclusively on the logical relations between theories and the evidence that bears upon them. The logic of scientific discovery, then, is not a logic of invention but a logic of justification, or more precisely, a logic of criticism. A scientist may arrive at a theory through a dream, a guess, or a flash of insight; what matters is how that theory is tested and evaluated.
The central logical concept here is falsification, which Popper distinguishes carefully from verification. Verification requires a positive logical connection between evidence and theory: the truth of observation statements must somehow guarantee the truth of the theory. This is impossible for universal theories. Falsification, by contrast, has a clean logical form. A single counter-instance logically falsifies a universal statement: if we observe one black swan, the statement “all swans are white” is logically refuted. The logical asymmetry between verification and falsification is absolute. Popper calls this the asymmetry thesis: universal statements can never be verified but can be falsified.
Yet falsification is not as simple as a single observation. Popper introduces the concept of basic statements — singular observation statements that describe an observable event (e.g., “there is a black swan in this pond at time t”). For a theory to be falsifiable, it must entail the negation of some class of basic statements. The falsifiability hierarchy emerges: some theories are more falsifiable than others because they rule out more possible observations. A theory that forbids nothing is useless. A theory that forbids much is bold and informative. The degree of falsifiability correlates with the empirical content of the theory.
Popper also addresses the problem of conventionalism, the view that scientific theories are merely convenient conventions for organizing observations and can never be genuinely refuted. Against this, Popper insists that scientists can decide to treat a theory as falsified — that is the methodological decision at the heart of science. But these decisions are not arbitrary: they are governed by rules of method that Popper will develop, and they can themselves be criticized. The critical attitude must extend even to the methods of criticism.
What this means for the reader is that Popper has constructed a complete logical framework for scientific rationality without any appeal to induction or confirmation. Science is rational not because it can prove its theories true, but because it can detect and eliminate errors. The scientist’s task is not to find evidence that supports a theory but to find evidence that might refute it. This inversion of traditional methodology is the core of Popper’s revolution.
“The game of science is, in principle, without end. He who decides one day that scientific statements do not call for any further test, and that they can be regarded as finally verified, retires from the game.”
Part Two: The Growth of Knowledge and the Method of Conjectures and Refutations
We now enter the section where Popper extends his logic of falsification into a dynamic theory of scientific progress. The static logic of falsification becomes the engine of knowledge growth.
Popper argues that scientific knowledge grows through a repeated cycle: problem → tentative theory → error elimination → new problem. This is the tetradic schema that he presents as the universal pattern of rational learning. Every inquiry begins with a problem — a contradiction between existing theories and observations, or between two theories, or a puzzle within a theory. The scientist then proposes a bold conjecture as a solution. But this conjecture cannot be accepted uncritically. It must be subjected to severe tests designed to expose its weaknesses. When the conjecture fails a test, we learn something: we eliminate an error, and we arrive at a new, deeper problem. The process repeats.
Popper insists that this schema applies not only to science but to all forms of learning. Even animal learning, he suggests, follows the pattern of trial and error elimination. The difference is that in science, the trials — the conjectures — are formulated in language and can be criticized publicly, without killing the scientist who proposed them. This is the great advantage of scientific method: theories can die in our place.
A crucial concept introduced in this section is verisimilitude, or truthlikeness. Popper recognizes that many scientific theories that we accept are not literally true — Newtonian mechanics was falsified by Einstein’s relativity, yet it was an immense advance over Aristotelian physics. How can we say that science makes progress if even our best theories are eventually replaced? Popper’s answer is that theories can have greater or lesser verisimilitude: a theory is closer to the truth if it has more true consequences and fewer false consequences than its predecessor. Even though we can never know that a theory is true, we can sometimes know that it is closer to the truth than another theory. The growth of knowledge is an asymptotic approach to truth.
Popper also addresses the problem of ad hoc hypotheses. A scientific community that protects its theories by adding untestable auxiliary assumptions is abandoning the critical attitude. A valid scientific theory must be independently testable: its predictions must go beyond the observations it was designed to explain. When a theory fails a test, the scientist may propose a new theory, but that new theory must make new testable predictions. Otherwise, it is merely a conventionalist stratagem — a trick to immunize the theory against refutation.
For the reader, this section reveals that Popper’s philosophy is not static but profoundly dynamic. Science is not a structure of justified beliefs but an ongoing process of conjecture and criticism. The goal is not certainty but progress — getting closer to the truth even if we never finally arrive. This is the heart of what Popper calls critical rationalism: the willingness to submit all claims, including one’s own most cherished theories, to relentless criticism.
“We do not know: we can only guess. And our guesses are guided by the unscientific, the metaphysical faith in laws, in regularities which we can uncover — in which we can discover.”
Part Three: The Demarcation Between Science and Metaphysics
We now move to the section where Popper refines his famous demarcation criterion and addresses the relationship between science and metaphysics. This is a crucial clarification, because Popper’s criterion is often misunderstood as a simple dismissal of all non-scientific statements as meaningless.
Popper begins by distinguishing his demarcation criterion from the verification criterion of meaning advanced by the Vienna Circle logical positivists. The positivists argued that any statement that cannot be empirically verified is meaningless — including metaphysical, ethical, and aesthetic claims. Popper rejects this position entirely. He insists that the demarcation between science and non-science is a matter of method, not of meaning. Metaphysical statements can be meaningful, important, and even fruitful for science, but they are not scientific because they are not falsifiable.
The key examples Popper examines are Marx’s theory of history, Freud’s psychoanalysis, and Adler’s individual psychology. These theories, Popper argues, were presented as scientific but were actually unfalsifiable. They could explain any possible human behavior by appealing to hidden mechanisms — the class struggle, the unconscious, or the inferiority complex. If a patient refused to accept the diagnosis, the psychoanalyst could interpret this refusal itself as a symptom of resistance, thereby protecting the theory from any possible refutation. Against this, Popper contrasts Einstein’s theory of relativity, which made specific, risky predictions — such as the bending of light by gravity — that could be tested by observation. The eclipse expedition of 1919, which confirmed Einstein’s prediction, is presented as the paradigm of genuine scientific testing.
Popper introduces the concept of metaphysical research programs. Even though metaphysical statements are not falsifiable, they can guide scientific inquiry by suggesting fruitful lines of research. For example, the metaphysical belief that the universe is governed by regular, discoverable laws — the principle of uniformity of nature — is not itself a scientific statement, but it motivates the search for such laws. Atomism was originally a metaphysical doctrine, but it eventually led to testable scientific theories. Popper thus maintains a productive relationship between science and metaphysics while keeping the boundary clear.
A further refinement concerns historical prediction. Popper argues that large-scale historical predictions — the claim that history follows inevitable laws, as in Marxism — are not truly scientific because they cannot be tested. History is not like physics; there are no experimental controls, and theories about history can always be saved by reinterpretation. This critique leads directly into his later work on the open society and the critique of historicism.
What this section accomplishes is the rescue of metaphysics from the positivist trash heap while preserving the integrity of science. Popper shows that meaningful discourse extends far beyond the boundaries of science. The critical rationalist does not dismiss metaphysical claims as nonsense but subjects them to rational criticism of a different kind — logical analysis, coherence, and fruitfulness for further inquiry. Science retains its special status not because it is the only source of meaning, but because it is the one domain where claims can be decisively tested by experience.
“The difference between Einstein and an amoeba is that Einstein consciously seeks error elimination. The amoeba cannot be critical; it cannot tell where it went wrong; it simply dies when it errs.”
Part Four: The Problem of the Empirical Basis
We now move from the demarcation between science and metaphysics to one of the most frequently misunderstood aspects of Popper’s philosophy: the problem of the empirical basis. This section addresses the question: if scientific theories cannot be verified, and if observation statements themselves are fallible, on what ground does science ultimately rest? Popper is here confronting the charge that his falsificationism leads to an infinite regress or a dogmatic foundation.
Popper begins by rejecting the two traditional solutions to the problem of the empirical basis. The first solution — psychologism — holds that perceptual experiences provide a secure foundation for knowledge. The second solution — conventionalism — holds that the empirical basis is simply agreed upon by convention, making science a matter of arbitrary decision. Popper finds both unacceptable. Psychologism confuses the causes of belief with the justifications for belief; conventionalism reduces science to a game without objective truth.
Popper’s own solution is distinctive and often misrepresented. He argues that the empirical basis of science consists of basic statements (also called test statements or observation statements). These are singular existential statements — for example, “There is a pointer at position 3.2 on the scale” or “There is a black swan in this pond.” Basic statements are not reports of subjective experiences; they are public, intersubjectively testable claims about observable states of affairs. They have the logical form of singular existential propositions, and they are the kind of statement that can serve as potential falsifiers for universal theories.
The crucial point is that basic statements themselves are not infallible. They are conventions in a specific sense: we decide to accept a basic statement provisionally, for the purposes of a test, without claiming that it is certainly true. This decision is not arbitrary — it is guided by intersubjective agreement among competent observers under appropriate conditions. But the decision remains fallible. As Popper puts it, basic statements have the status of “decisions arrived at by agreement” that are always open to revision if new evidence or new criticism emerges.
Here Popper introduces his famous “sieve” metaphor. The empirical basis is like a sieve that filters out falsified theories, but the holes of the sieve — the basic statements themselves — are not permanently fixed. They can be made finer or coarser as our observational techniques improve. The acceptance of a basic statement is always provisional and relative to the state of our knowledge. We do not ground science on indubitable foundations; we ground it on a constantly moving and self-correcting empirical basis.
This leads to a remarkable consequence: the empirical basis does not provide certainty but testability. Science does not rest on a bedrock of unquestionable facts; it rests on a marsh that is constantly being drained and refilled. Popper compares the process to building a house on piles driven into a swamp: the piles never reach solid ground, but as long as they are driven deeper and connected together, the house remains stable enough for practical purposes. The stability comes not from absolute foundations but from the critical process itself — the willingness to question every basic statement and replace it with a better one when necessary.
“The empirical basis of objective science has thus nothing ‘absolute’ about it. Science does not rest upon solid bedrock. The bold structure of its theories rises, as it were, above a swamp. It is like a building erected on piles. The piles are driven down from above into the swamp, but not down to any natural or ‘given’ base; and when we stop our attempts to drive our piles into a deeper layer, it is not because we have reached firm ground. We simply stop when we are satisfied that they are firm enough to carry the structure, at least for the time being.”
What Popper has done is to replace the traditional foundationalist picture of knowledge with a fallibilist one. The empirical basis is not the source of certainty but the point of contact between our theories and reality — a contact that is always mediated by human decisions, theoretical assumptions, and intersubjective agreement. The objectivity of science is not secured by the indubitability of observations but by the public and critical character of the testing process. Any basic statement can be challenged, and any challenge must itself be settled by reference to further basic statements, in a process that never reaches an absolute endpoint.
This position has been widely criticized as a form of conventionalism — the very view Popper claimed to reject. But Popper’s reply is that the conventions involved are not arbitrary. They are guided by the critical method itself: we accept basic statements provisionally only when they have survived attempts to falsify them through intersubjective testing. The decision to accept a basic statement is a resolution reached through rational discussion, not a stipulation imposed by fiat. The difference between Popper’s view and conventionalism proper is that for Popper, conventions are always open to revision in light of new criticism, whereas for the conventionalist they are ultimate and unchangeable.
Part Five: Corroboration and the Degrees of Testability
Having established the fallible empirical basis, Popper now turns to a concept that has caused immense confusion among his readers: corroboration. This section is essential because it addresses the obvious question: if theories can never be verified, and if falsification only tells us when a theory is wrong, how do we decide which theories are better? How does science make progress toward truth?
Popper begins by distinguishing corroboration from verification and probability. Corroboration is not a measure of how likely a theory is to be true; it is a report of how well a theory has survived testing so far. A highly corroborated theory is one that has been subjected to severe tests and has withstood them. But this does not mean the theory is probable or likely to be true in the future. As Popper insists, “corroboration is not a measure of the probability that a theory will survive further tests.” It is a historical report on past performance, not a prediction of future success.
The degree of corroboration depends on two factors: the severity of the tests the theory has passed, and the number of independent tests it has survived. A theory that passes only trivial tests — tests that any reasonable alternative would also pass — is not highly corroborated. A theory that passes severe tests — tests that would very likely falsify the theory if it were false — earns a high degree of corroboration. This connects directly to Popper’s earlier discussion of testability: a theory’s degree of testability is determined by its empirical content, which is measured by the class of potential falsifiers it rules out. The more a theory forbids, the more testable it is, and the more severely it can be tested.
Popper formalizes this with his concept of logical probability versus informational content. A theory with high logical probability (close to 1) says very little — it is vague and cautious. A theory with low logical probability (close to 0) says a great deal — it is precise and bold. Popper’s slogan is: the content of a theory increases with its degree of falsifiability and decreases with its logical probability. This means that the most interesting and valuable theories are those that are highly improbable from a logical point of view — they take the greatest risks. The criterion of progress in science is therefore not increasing probability but increasing testability and corroboration through severe testing.
This leads Popper to a striking reversal of traditional epistemology. Most philosophers have held that the goal of science is to achieve probable or likely truths. Popper argues that this goal is misguided. If we want probable theories, we should adopt vague, cautious theories that say almost nothing — theories with high logical probability. But such theories are useless for science because they tell us little about the world. The real goal of science is to find bold, highly falsifiable theories that take risks and then survive severe testing. The truth of a theory matters, but we can never know with certainty that a theory is true; we can only know that it has passed the most stringent tests we can devise.
“The degree of corroboration of a theory is not a measure of its verisimilitude. It does not tell us how close to the truth the theory is, but only how well it has stood up to criticism so far. The difference is important: a highly corroborated theory may be false, and a poorly corroborated theory may be true.”
Popper distinguishes sharply between corroboration and verisimilitude (truthlikeness). Verisimilitude is an objective relation between a theory and the truth — how much of the truth the theory captures and how much falsehood it contains. Corroboration is a subjective evaluation based on the evidence available to us. We cannot infer from high corroboration that a theory has high verisimilitude; all we can say is that, so far, the theory has survived all attempts to show it is false. This distinction is crucial because it prevents Popper from falling into a naive optimism that equates scientific success with truth.
The practical upshot is that scientists should prefer theories that are highly testable, bold in their predictions, and severely tested. A theory that has survived many severe tests is better corroborated than one that has only passed trivial tests, but this does not mean it is closer to the truth. It simply means it has earned a provisional right to be preferred for practical purposes — for example, in technological applications or as a basis for further research. The preference rule is pragmatic, not epistemic: we prefer the best-corroborated theory not because it is certainly true but because it has shown itself to be the most resistant to criticism so far.
Part Six: The Theory of Truth and Verisimilitude
We now arrive at a part of Popper’s work that represents a major development in his thinking. In his earlier writings, Popper was primarily concerned with the logic of falsification and the growth of knowledge. But critics pointed out that without a concept of truth, his account seemed to reduce science to a series of increasingly better-tested falsehoods. In response, Popper turned to Tarski’s semantic theory of truth and developed his own concept of verisimilitude to give content to the idea of scientific progress toward truth.
Popper begins by acknowledging that his earlier work had been “suspicious of the very idea of truth” because of the philosophical problems associated with it — the threat of relativism, the difficulty of defining truth, and the misuse of truth claims by dogmatists. But reading Alfred Tarski’s work convinced Popper that a correspondence theory of truth could be stated with perfect logical rigor, without falling into metaphysical confusion. Tarski showed that truth is a property of statements: a statement is true if and only if the world is as the statement says it is. “Snow is white” is true if and only if snow is white. This seems trivial, but Tarski’s achievement was to show that this intuition can be formally stated without paradox.
Popper enthusiastically adopts Tarski’s theory. He calls it the “greatest philosophical achievement of the twentieth century” because it shows that the concept of truth as correspondence is not vague or mystical but can be made precise and objective. More importantly for Popper’s project, Tarski’s theory is realist: it assumes a world independent of our beliefs and statements, and it defines truth in terms of the relation between language and reality. This gives Popper the tool he needs to talk about objective knowledge — knowledge that is true regardless of what anyone believes about it.
But Popper recognizes a serious problem. If we can never know with certainty that a theory is true — if all our theories are, strictly speaking, conjectures that may turn out to be false — then what sense does it make to talk about progress toward truth? How can we say that Einstein’s theory is closer to the truth than Newton’s if both are false? (Newton’s theory is false because it fails for high velocities and strong gravitational fields; Einstein’s theory is also false because it fails to account for quantum phenomena.) Popper needs a concept of greater or lesser truthlikeness — verisimilitude — that allows us to compare false theories.
Popper’s definition of verisimilitude is based on the truth-content and falsity-content of a theory. The truth-content of a theory is the set of all true statements that follow from it. The falsity-content is the set of all false statements that follow from it. One theory has greater verisimilitude than another if: (1) its truth-content is greater (it entails more true statements), (2) its falsity-content is not greater (it does not entail more false statements), and (3) the increase in truth-content is not entirely due to an increase in falsity-content that cancels it out.
The intuitive idea is this: when Einstein’s theory was proposed, it preserved all the true predictions of Newton’s theory (for the domain where Newton worked) while also making new true predictions (such as the bending of light by gravity) that Newton’s theory could not make. It also failed to preserve some of Newton’s false predictions (such as the precession of Mercury’s perihelion, which Newton’s theory got wrong). So Einstein’s theory has greater truth-content and less falsity-content than Newton’s — it is closer to the truth, even though strictly false.
“We can say of a theory that it is nearer to the truth than another if, and only if, it has more truth content than the other theory, and no more falsity content. This is a purely logical definition, and it does not require any knowledge of the absolute truth of the theories compared.”
Popper immediately acknowledges a devastating problem with this definition. It turns out that under plausible interpretations of “truth-content” and “falsity-content,” the definition allows that a theory with greater verisimilitude can be created simply by adding false statements to a true theory. This is because the falsity-content paradoxically seems to grow more slowly than the truth-content when new false statements are added. Popper struggled with this problem for decades and never fully solved it. The definition of verisimilitude remains one of the most controversial and contested parts of his philosophy.
Despite this technical difficulty, the concept of verisimilitude is essential to Popper’s overall project. It allows him to maintain that science makes genuine progress — not just by accumulating better-tested theories, but by moving toward a more complete and accurate representation of reality. Even though we can never know when we have reached the truth, we can legitimately claim that later theories are objectively better than earlier ones in terms of their truthlikeness. This gives Popper’s critical rationalism a realist backbone: science is not just a game of conjecture and refutation but a genuine search for truth, guided by rational criticism and empirical testing.
The importance of verisimilitude for Popper’s system cannot be overstated. Without it, his philosophy collapses into a form of instrumentalism — the view that theories are merely useful tools for prediction, not true or false representations of reality. With it, Popper can claim that science aims at truth, that we can make progress toward it, and that this progress is both objective and rationally assessable. The technical problems with the definition are real, but the program of defining verisimilitude remains philosophically vital: it captures the intuition that science improves over time in a way that is not merely pragmatic but genuinely cognitive.
Part Seven: The Problem of Objective Knowledge and the Three Worlds
We move from the theory of truth and verisimilitude to Popper’s broader epistemological framework: the theory of objective knowledge and his distinctive three worlds ontology. This section extends his philosophy of science into a general theory of knowledge, addressing the question of what knowledge is, where it resides, and how it grows.
Popper begins by distinguishing sharply between subjective knowledge — knowledge in the sense of mental states, beliefs, or dispositions — and objective knowledge — knowledge in the sense of propositions, theories, and arguments that exist independently of any knowing subject. He argues that traditional epistemology has been dominated by what he calls the “belief philosophy” or subjectivist approach, which treats knowledge as a special kind of justified true belief held in the mind. Against this, Popper insists that the proper object of epistemology is not mental states but the logical content of theories and their relationships to one another and to the problems they attempt to solve.
This distinction leads directly to Popper’s three worlds ontology. World 1 is the physical world of material objects, forces, and states. World 2 is the mental world of subjective experiences, thoughts, feelings, and consciousness. World 3 is the world of objective knowledge — the world of theories, problems, arguments, propositions, and cultural artifacts that have been produced by human minds but have acquired a degree of autonomy from their creators. Popper writes that “the idea of a third world of objective knowledge — a world of the logical contents of our theories, our conjectures, our problems — is the central idea of my epistemology.”
“We are not students of some subject matter, but students of problems. And problems may cut right across the borders of any subject matter or discipline.”
The content of World 3 includes not only scientific theories but also works of art, mathematical entities, legal systems, and even the logical content of false theories. What makes something a World 3 object is its objectivity — its existence as a public, criticizable entity that can be discussed, evaluated, and improved upon independently of any particular individual’s mental states. A mathematical proof, for example, exists in World 3; its discovery or understanding by a particular mathematician belongs to World 2. The proof can be checked, criticized, and revised without reference to the mental processes of its discoverer.
Popper argues that World 3 objects are real in a meaningful sense — they causally interact with World 1 and World 2. A scientific theory (World 3) can lead to technological innovations (World 1) by way of the understanding and application by scientists (World 2). Conversely, physical instruments (World 1) can lead to new observations that result in theoretical revisions (World 3) through the mediation of scientists’ mental processes (World 2). This interactionist view is crucial: it allows Popper to maintain the autonomy of objective knowledge while acknowledging its dependence on human minds for its creation and its causal efficacy in the physical world.
The clarification here is that Popper’s three worlds theory is not merely a taxonomic exercise but a fundamental reorientation of epistemology. By locating knowledge primarily in World 3 rather than World 2, Popper dissolves many classic philosophical problems about skepticism, justification, and the relationship between mind and world. The growth of knowledge becomes a process of World 3 evolution — the creation, critique, and improvement of objective theories — rather than a process of accumulating mental certainties. This shift from a subjective to an objective epistemology is one of the most radical aspects of Popper’s philosophy, and it connects directly to his commitment to fallibilism and critical rationalism.
Part Eight: The Evolutionary Epistemology
From the three worlds ontology, Popper develops a comprehensive evolutionary epistemology that links the growth of scientific knowledge to biological evolution. This section presents his argument that knowledge, at both the biological and scientific levels, grows through a process of variation and selective elimination — a pattern he calls the “general theory of evolution” or “the evolutionary theory of knowledge”.
Popper begins by observing that all living organisms are problem-solvers. Every organism faces problems of survival, and its behavior, structure, and even its genetic makeup can be understood as conjectural solutions to these problems. A bacterium swimming toward a nutrient source, a plant growing toward light, an animal developing camouflage — all are instances of knowledge in a broad sense, where “knowledge” means adaptive expectation or disposition to behave as if certain regularities hold. This knowledge is embodied in the organism’s structure and behavior, not necessarily in any conscious or propositional form.
The mechanism of growth in this biological knowledge is variation and selective elimination. Random genetic mutations (variations) are tested against the environment (selection), and those that prove adaptive are retained. Popper emphasizes that this process is fundamentally hypothetical and fallible — no adaptation is perfect or permanent; all are subject to future error and revision. He states that “from the amoeba to Einstein, the growth of knowledge is always the same: we try to solve our problems, and to obtain, by a process of elimination, something approaching adequacy in our tentative solutions.”
“The growth of all knowledge consists in the modification of earlier knowledge — by its correction, by its improvement, or by its replacement.”
The difference between the amoeba and Einstein, according to Popper, is one of method rather than fundamental logic. The amoeba cannot criticize its hypotheses; it can only live or die by them. Einstein, by contrast, can formulate his hypotheses in language (a World 3 object), subject them to rational criticism, and allow them to die in his stead. This is the crucial advantage of conscious critical thinking: it allows hypotheses to be eliminated without the organism being eliminated. Popper calls this “exosomatic” knowledge — knowledge that exists outside the body in the form of theories, books, and other artifacts.
Popper extends this evolutionary framework to the growth of scientific knowledge specifically. Scientific theories are conjectures — bold hypotheses about the structure of reality that go far beyond the available evidence. These conjectures are then subjected to severe tests — attempts to refute them through observation and experiment. Those that survive testing are corroborated, but never verified. The process is continuous: even well-corroborated theories may eventually be replaced by better ones that survive even more severe tests. This is why Popper describes science as “permanent revolution” — a process of constant conjecture and refutation, with no final resting point.
The importance of this evolutionary epistemology for Popper’s system is that it naturalizes the growth of knowledge without reducing it to mere biology. By showing that the same logic of trial and error operates at every level — from genetic mutation to scientific revolution — Popper provides a unified account of knowledge that bridges the gap between the natural and the human sciences. At the same time, the distinction between unconscious adaptation and conscious criticism preserves the special role of rationality and World 3 in human knowledge. The evolutionary framework reinforces Popper’s fallibilism: if all knowledge is conjectural, subject to elimination, and never final, then the search for certainty is not merely impossible but conceptually misguided.
Part Nine: The Open Society and Its Enemies
Having established his epistemological foundations, Popper now turns to their implications for social and political philosophy. This section — one of the most famous and influential parts of his work — applies critical rationalism to the analysis of society, history, and politics, developing the contrast between open societies and closed societies and subjecting historicism to devastating critique.
Popper defines the closed society as a tribal or magical society in which social customs, laws, and institutions are regarded as fixed, sacred, and beyond criticism. In such a society, the individual’s place is determined by tradition, authority, and collective identity. Dissent is not merely forbidden but unthinkable — the very idea that one might question the fundamental norms of society is inconceivable. Popper identifies this with the “organic” or “tribal” form of social life, where the distinction between natural and conventional laws is blurred or nonexistent.
The open society, by contrast, is one in which individuals are free to make their own decisions, to criticize existing institutions, and to participate in the rational discussion of social and political questions. It is characterized by critical debate, individual responsibility, and the recognition that social institutions are human creations that can be rationally evaluated and reformed. Popper argues that the transition from the closed to the open society is one of the most profound and painful revolutions in human history, and that it is by no means irreversible.
“The open society is one in which individuals are confronted with personal decisions.”
The central target of Popper’s political critique is historicism — the doctrine that history follows a predetermined pattern or law that can be discovered and used to predict the future. He distinguishes historicism from mere historical prophecy (predicting future events based on current trends) and instead defines it as the claim that there are historical laws or developmental stages that societies must pass through, and that knowledge of these laws enables us to foresee and perhaps accelerate the inevitable course of history.
Popper identifies two main variants of historicism. The first is Platonic historicism, which he analyzes in detail through Plato’s political philosophy in the *Republic*. Popper argues that Plato’s ideal state is a reaction against the open society of Athens — a nostalgic attempt to return to the closed, tribal society of the past by imposing a rigid class hierarchy and suppressing critical thought. The second variant is Marxist historicism, which claims to have discovered the economic laws of historical development and predicts the inevitable overthrow of capitalism and the establishment of communism. Popper treats both as totalitarian doctrines because they justify oppression in the name of historical necessity — if history is on your side, any means are justified to achieve its predetermined end.
Against historicism, Popper proposes piecemeal social engineering — the method of small, gradual, and testable reforms that can be evaluated and corrected through critical discussion. This approach is the social analogue of scientific method: instead of attempting to redesign society according to a grand blueprint, the piecemeal engineer identifies specific social problems (poverty, crime, inequality, educational failures) and proposes limited, reversible interventions to address them. The results are then monitored, criticized, and adjusted as needed.
The clarification here is that Popper’s political philosophy is not a separate project from his epistemology but a direct application of critical rationalism to social life. The same fallibilism, the same rejection of authority, the same emphasis on criticism and testing that characterizes scientific method also characterizes the open society. The closed society is the social embodiment of justificationism — the belief that certain institutions or authorities are beyond criticism. The open society is the social embodiment of critical rationalism — the recognition that all institutions, all policies, all traditions are fallible and open to improvement. This is why Popper calls his political philosophy “critical liberalism” or “liberalism of the open society” — a liberalism grounded not in individual rights or economic freedom but in the epistemological priority of criticism over justification.
Part Ten: The Poverty of Historicism
We are in the second major application of critical rationalism to social and political philosophy. In *The Poverty of Historicism*, Popper attacks the doctrine that historical prediction is the proper aim of the social sciences, and he does so using the epistemological framework already established: the impossibility of justifying predictions from past regularities, the asymmetry between testability and falsification, and the role of criticism as the motor of knowledge growth.
Popper begins by defining historicism as “an approach to the social sciences which assumes that *historical prediction* is their principal aim, and which assumes that this aim is attainable by discovering the ‘rhythms’ or the ‘patterns’, the ‘laws’ or the ‘trends’ that underlie the evolution of history.” The historicist believes that history has a direction, that this direction can be discovered, and that once discovered it permits long-term predictions about the future course of social development. Popper identifies this view in thinkers as diverse as Plato, Hegel, Marx, Comte, Mill, and Spengler. Despite their profound disagreements, they share the fundamental assumption that large-scale historical laws exist and can be used for prediction.
The core of Popper’s argument against historicism is epistemological, not historical. He argues that the growth of human knowledge is a decisive factor in the course of history, and that we cannot predict, by rational or scientific methods, the future growth of our own knowledge. This is because if we could predict what we will know tomorrow, we would already know it today. The argument is crisp and devastating: “We cannot, therefore, predict the future course of human history.” This is not a practical limitation but a logical one. The prediction of future knowledge is self-defeating — any prediction of a future scientific discovery would require that the prediction itself constitute that discovery, making the prediction either false or trivial.
Popper distinguishes between scientific prediction and historical prophecy. Scientific prediction, as in physics or astronomy, is conditional: “If X happens, then Y will follow.” Historical prophecy, as in historicism, is unconditional: “History will move toward a classless society” or “The West will decline.” Scientific predictions are tested by experiments that can falsify them; historical prophecies are immunized against falsification by their vagueness, their time-scales, and their appeal to inevitability. The historicist treats his prophecies as if they were scientific laws, but they lack the conditional form and the testability that genuine scientific laws possess.
“The historicist does not recognize that it is we who select and order the facts of history. Instead, he believes that it is the ‘history itself,’ or the ‘history of mankind,’ which, by its inherent laws, determines itself, and determines us, our problems, our future, even our point of view.”
Popper then turns to the methodological implications. Historicism, he argues, leads to a misunderstanding of social institutions. It treats them as if they are natural objects obeying natural laws, rather than human creations that can be reformed. This produces a “moral futurism” — the belief that whatever is coming is good because it is coming, and that we must adapt ourselves to the inevitable. It also produces a passive, fatalistic attitude toward social problems. If history has laws that determine the future, then political action is either pointless (if it cannot change what must happen) or dangerous (if it attempts to resist the inevitable). Popper contrasts this with the attitude of “piecemeal social engineering” — the view that we can identify specific social problems, propose solutions, test them in practice, learn from our mistakes, and gradually improve our institutions without claiming to know the final destination of history.
The clarification here is that Popper’s argument against historicism is not a rejection of social science or of social prediction. It is a rejection of the claim that the social sciences can discover the laws of historical development. Social science can and should make conditional predictions about the consequences of specific policies; it can and should analyze the logic of social situations; it can and should criticize existing institutions. What it cannot do is predict the course of history as a whole, because that would require predicting the future state of knowledge, which is logically impossible. The poverty of historicism is therefore the poverty of a method that promises more than any method can deliver — and that, in promising too much, undermines the conditions for rational social reform.
Part Eleven: The Open Society and Its Enemies — Plato
We are now inside the first volume of Popper’s most famous political work. *The Open Society and Its Enemies* applies critical rationalism to the history of political philosophy, arguing that Plato, Hegel, and Marx each developed philosophical systems that justified totalitarianism or its theoretical preconditions. Popper begins with Plato not only because he is historically first, but because Plato’s political philosophy is the most sophisticated and influential defense of the closed society ever written.
Popper defines the closed society as a tribal or magical society in which social roles are fixed, questioning is discouraged, and the individual is subordinated to the collective. The open society, by contrast, is one in which individuals are free to make personal decisions, criticize institutions, and participate in rational discussion about the direction of social life. The transition from closed to open society, Popper argues, began in ancient Athens with the rise of democracy and the critical tradition associated with Socrates. Plato’s political philosophy is best understood as a reaction against this transition — a systematic attempt to arrest it and return to the stability and authority of the closed society.
“Plato’s demand for the unity of the state, his utopian program, is a demand for the total abolition of all change; it belongs to the same mood as the demand for the stoppage of all social change, for the permanence of a social system, that is, for the closed society.”
Popper traces this through three interconnected lines in Plato’s thought. First, the theory of Forms is pressed into political service. Just as there is an ideal Form of justice that is unchanging and perfect, there is an ideal Form of the state that must be imitated without alteration. Change is decay; deviation from the Form is corruption. Political knowledge consists in knowing this unchanging ideal, not in learning from trial and error. Second, the theory of the soul is mapped onto the theory of the state: the rational part of the soul corresponds to the philosopher-kings, the spirited part to the guardians, the appetitive part to the producers. Justice is the harmony in which each part does its own work without interfering with the others. This is a recipe for rigid hierarchy. Third, the generational decay of regimes (from timarchy to oligarchy to democracy to tyranny) is presented as an inevitable historical law. The only way to stop this decay is to prevent any change from occurring at all — to freeze the political order at its best moment.
Popper argues that Plato’s attack on democracy is not merely conservative but totalitarian. Democracy is condemned precisely because it allows change, criticism, and the free play of opinions. Socrates was condemned by the democracy, but this does not justify Plato’s response — to condemn democracy itself and to propose a state in which the rulers are permitted to lie (“the noble lie”) to maintain social order, to practice eugenics, to control education, to exile or kill those who question the regime. The open society is a society of individuals who have learned to live with uncertainty, criticism, and the burden of personal responsibility. Plato offered escape from that burden into the security of a closed, hierarchical, unchanging order. His philosophy is therefore, in Popper’s judgment, the first and most dangerous enemy of the open society.
The clarification is that Popper is not merely offering a political critique of Plato; he is reading Plato’s epistemology as continuous with his politics. Plato’s theory of knowledge as justified true belief (the *Theaetetus* definition) and his view that genuine knowledge is of unchanging Forms that can be known with certainty, is the epistemological counterpart of his political authoritarianism. The claim to know the ideal state with certainty justifies the imposition of that state on those who do not know. The critical rationalist epistemology of fallibilism, by contrast, is the epistemological counterpart of the open society: if no one can claim certain knowledge of the good society, then everyone must be free to propose, to criticize, and to test their proposals. The open society is not built on agreement about final ends but on agreement about the procedures for criticizing and revising our proposals.
Part Twelve: The Open Society and Its Enemies — Hegel and Marx
We are now in the second volume of *The Open Society and Its Enemies*. Popper moves from Plato to Hegel and Marx, presenting them as the second and third waves of the revolt against freedom. Hegel is treated with particular harshness: Popper calls his philosophy “the apotheosis of the closed society” and describes his influence as “the most pernicious in the history of thought.” The grounds for this judgment are both intellectual and political.
Popper argues that Hegel’s dialectic is a degenerate version of the critical method. The genuine critical method involves proposing a theory, criticizing it, and revising it in light of criticism. Hegel’s dialectic replaces this rational process with a metaphysical schema in which thesis, antithesis, and synthesis are categories of being itself. Criticism is not a human activity but a cosmic necessity. Contradiction, far from being the sign of a false theory (as it is in logic), becomes the motor of historical progress. This permits Hegel to treat any existing state of affairs as rational simply because it has emerged from the dialectical process. His famous dictum — “What is rational is actual, and what is actual is rational” — becomes, in Popper’s reading, a justification for the Prussian state and for the suppression of criticism. The open society depends on the recognition that what exists can be criticized; Hegel’s philosophy makes criticism of what exists into a contradiction in terms.
“Hegel’s dialectic is a method of avoiding criticism. It is a brand of intellectual dishonesty which is unfortunately typical of much of the philosophy of our time. By equating existence with rationality, it destroys the basis of any critical evaluation of the existing order.”
Marx, for Popper, is a far more serious figure. Popper acknowledges Marx’s intellectual power, his genuine scientific contributions to economics and sociology, and his passionate commitment to human liberation. Marx’s attack on capitalism, his analysis of alienation, his theory of class struggle, and his critique of ideology are all important contributions to the understanding of social life. What Popper rejects is Marx’s historicism — the claim that he had discovered the laws of historical development that would inevitably lead to socialism, and that these laws permitted scientific prediction of the future. This, for Popper, turned Marxism from a critical science into a dogmatic religion.
Popper identifies the critical moment in Marx’s thought. Marx believed that he had turned Hegel’s dialectic “right side up” by making it materialist rather than idealist. The dialectic of history is driven not by the self-development of Spirit but by the conflict between the forces and relations of production. This generates class conflict, revolution, and the eventual creation of a classless society. Popper’s critique is not that this is empirically false (though he believes parts of it are), but that it is methodologically illegitimate. The predictions of Marxism are not testable in the way that scientific predictions should be. When predictions fail, Marxists do not abandon the theory; they reinterpret it, add auxiliary hypotheses, or explain the failure as the result of a “temporary setback.” The theory thus becomes immunized against falsification. It ceases to be science and becomes what Popper calls a “pseudo-science” — a system that appears to explain everything but actually explains nothing because it can be adjusted to fit any evidence.
Popper develops this critique in detail through Marx’s analysis of the coming revolution. Marx predicted that capitalism would become increasingly polarized, with the proletariat growing larger and poorer while the bourgeoisie grew smaller and richer. This did not happen; the working class in advanced capitalist societies became more prosperous and more integrated into the political system. Marx predicted that revolutions would occur in the most advanced capitalist countries; they occurred in the least developed ones. Marx predicted that the state would wither away after the revolution; it became more powerful. Each of these failures could be explained away, but the very ability to explain them away is the problem. A genuinely scientific theory would have been revised or abandoned in the face of such disconfirming evidence. Marxism, in its refusal to do so, showed itself to be not a science but a historicist prophecy.
The clarification is that Popper’s critique of Marx is not a defense of capitalism. Popper describes himself as a “social liberal” or “welfare liberal” who believes in piecemeal social reform to alleviate suffering and inequality. What he opposes is the revolutionary claim to know the final goal of history and to justify any means — including violence, dictatorship, and the suppression of criticism — as necessary for reaching that goal. Marx’s historicism, in Popper’s view, led directly to authoritarian Marxism, just as Plato’s authoritarian epistemology led to his totalitarian state. The enemy of the open society is not any particular economic system but the claim to have achieved certain knowledge of the ideal social order. The open society is the society that has learned to live without such certainty — to conduct its politics as a form of trial and error, to treat all institutions as provisional and open to criticism, and to value freedom of discussion above agreement about ends.
Part Thirteen: The Myth of the Framework
We are now in a later essay collection, where Popper refines and defends his position against misunderstandings. The essay “The Myth of the Framework” addresses the claim that rational discussion is impossible across different conceptual frameworks or paradigms — a claim that had become influential in the philosophy of science through the work of Thomas Kuhn and others. Popper argues that this claim, which he calls “the myth of the framework,” is itself a form of relativism that leads to irrationalism and undermines the possibility of criticism.
The myth of the framework holds that we are trapped within our own conceptual schemes, that we can only understand what accords with our own assumptions, and that communication across frameworks is impossible. If true, this would destroy the possibility of critical rationalism, since criticism depends on the possibility of comparing different theories, learning from alternative perspectives, and being persuaded by argument. Popper responds that the myth is demonstrably false. “We can communicate across frameworks,” he writes, “because we can learn to understand each other’s frameworks, and we can criticize them.” The very fact that we can recognize a framework as different from our own already shows that we are not trapped within it.
“The myth of the framework is, in our time, the bulwark of irrationalism. If the myth of the framework were true, there would be no rational discussion between different frameworks, no critical discussion; there would be no growth of knowledge.”
Popper argues that the myth is self-refuting. The person who claims that all frameworks are incommensurable is making a claim that is supposed to hold across frameworks — that is, the claim is supposed to be true for all frameworks, not just for the speaker’s own. But if frameworks are incommensurable, then the claim cannot be assessed by those in other frameworks. It becomes a piece of dogmatic assertion, not a contribution to rational discussion. Moreover, the history of science, philosophy, and culture demonstrates that cross-framework communication is not only possible but productive. When Einstein replaced Newton, the two frameworks were profoundly different, but the comparison was possible precisely because they shared enough common ground — such as the demand for mathematical precision and empirical testability — to make rational choice between them possible.
Popper develops this into a broader defense of critical rationalism against relativism. Relativism is the view that truth is relative to a framework, that what is true for one person or culture may not be true for another. Popper rejects this on two grounds. First, it confuses truth with certainty. We may never be certain that we have the truth, but this does not mean that truth varies with perspective. A theory is true if it corresponds to the facts, regardless of whether anyone believes it or can justify it. Second, relativism makes criticism meaningless. If all views are equally true relative to their frameworks, then there is no reason to prefer one theory over another, and no reason to test, criticize, or improve our theories. The myth of the framework, if taken seriously, would bring the growth of knowledge to a halt.
The clarification is that Popper’s rejection of the myth is not a naive claim that communication is easy or that frameworks do not matter. He fully acknowledges that different frameworks can make communication difficult, that assumptions can be deeply embedded, and that much effort is required to understand an alternative point of view. What he denies is that these difficulties are insurmountable in principle. The critical rationalist approach to frameworks is the same as the critical rationalist approach to theories: we propose, we criticize, we compare, we learn. The fact that we can be imprisoned by our assumptions does not mean we cannot also break out of them through the effort of criticism and the openness to new ideas. The myth of the framework is dangerous precisely because it tells us this effort is futile when it is not.
Part Fourteen: The Self and Its Brain — An Interactionist Epistemology
We are now in the collaborative work with John Eccles, *The Self and Its Brain*, where Popper extends his three-worlds epistemology to the problem of consciousness. The work is an extended argument for interactionism — the view that the mind and brain are distinct but interacting realities — as opposed to materialism, epiphenomenalism, or identity theory. Popper’s interest is not primarily in neuroscience but in the epistemological implications of a non-reductive account of mind.
Popper argues that World 3 (the world of objective knowledge, theories, arguments, works of art) cannot be reduced to World 1 (the physical world) or World 2 (the subjective world of consciousness). Theories are real; they have consequences; they interact with World 2 (through human understanding) and through World 2 with World 1 (through the physical actions of human beings who read and write and build). But the reality of World 3 is not physical. The Pythagorean theorem is not a physical object; it is a piece of abstract knowledge that can be discovered, understood, criticized, and applied. The physical marks on paper or the neural firings in a brain are not the theorem; they are the physical embodiments of the theorem. The theorem itself belongs to World 3.
This has direct implications for the problem of consciousness. If World 3 is real and non-reducible, then the human mind — which has access to World 3 — cannot be simply identical to the brain. The brain is a physical object; the mind is a system of World 2 states that can grasp World 3 objects. The mind interacts with the brain, but it is not the same thing. Popper defends interactionism against the charge of dualism by arguing that interaction is not a mysterious metaphysical postulate but an empirical fact: we experience it every time we think about a problem. The thought (World 2) is about a theory (World 3); the thought leads to changes in the brain (World 1); the brain changes lead to speech or writing; the speech or writing produces new World 3 objects that can be criticized by others. This causal chain is not mysterious; it is the ordinary process of rational thought.
“The mind is an active agent, not a passive recipient. It does not merely react to physical stimuli; it selects, interprets, criticizes, and creates. The brain is the instrument of the mind, not its creator.”
Popper argues that materialism about the mind is self-defeating in the same way that relativism is self-defeating. If all mental states are simply physical states of the brain, then the mental state of believing that materialism is true is simply a physical state of the materialist’s brain. But then the truth of materialism does not depend on argument or evidence; it depends solely on the physical state of the brain. If a materialist tries to argue for materialism, he is assuming that argument — a rational process involving the evaluation of evidence — can change beliefs. But if beliefs are just physical states, they are changed by physical causes, not by reasons. Materialism, in other words, cannot account for the rationality of argument. Interactionism, by contrast, allows that reasons can have causal power: a good argument (a World 3 object) is grasped by the mind (a World 2 agent), which then reorganizes the brain (World 1) accordingly.
The clarification is that Popper’s interactionism is not a return to Cartesian dualism. Descartes held that mind and body are two distinct substances that interact miraculously in the pineal gland. Popper rejects the notion of substance altogether. The mind is not a thing; it is a system of processes, states, and capacities that emerge from the brain’s activity but are not reducible to it. The brain, similarly, is not the mind’s prison; it is the medium through which the mind engages with the world. Interactionism is not a solution to all problems of consciousness; it is an acknowledgment that the reductive materialism of much neuroscience cannot account for the phenomena of rationality, creativity, and self-awareness that characterize human knowledge. The open society requires autonomous individuals capable of rational criticism; interactionism provides the metaphysical framework for understanding how such individuals are possible.
Part Fifteen: The Myth of the Framework — The Rationality of Scientific Revolutions
We are now in a further development of Popper’s response to Kuhn and the sociology of scientific knowledge. The essay “The Rationality of Scientific Revolutions” argues that scientific revolutions — the transition from one paradigm to another — are not irrational leaps of faith (as Kuhn sometimes suggested) but are, in principle, rational decisions governed by the logic of critical testing and the growth of knowledge.
Popper distinguishes between normal science (the puzzle-solving activity that takes place within an accepted framework) and revolutionary science (the period in which a framework is overthrown and replaced). He agrees with Kuhn that normal science exists and that it has a sociological reality: scientists do work within accepted paradigms, and they do resist change. But he denies that the transition from one paradigm to another is a matter of “conversion” or “gestalt switch” that cannot be rationally justified. The critical method — the method of proposing conjectures and attempting to refute them — applies to paradigms as much as to ordinary theories.
“The choice between competing paradigms is a rational choice, not because the paradigms can be compared by a common standard, but because the participants in the dispute can learn from each other, can criticize each other, and can come to see the superior explanatory power of one paradigm over another.”
Popper argues that the rationality of paradigm choice depends on the critical comparison of the competing frameworks. Einstein’s theory did not replace Newton’s theory because of a sociological shift; it replaced it because Einstein’s theory made more precise predictions, survived more severe tests, and explained phenomena (such as the precession of Mercury) that Newton’s theory could not explain. The fact that different scientists may weigh these criteria differently does not mean that the decision is arbitrary. It means that rational discussion is possible: one scientist can argue that explanatory power is more important than simplicity; another can argue the opposite. The discussion itself is governed by shared norms of argument, evidence, and criticism.
Popper also addresses the objection that paradigms are incommensurable — that they cannot be compared because they use different concepts, different standards, and different methods. He argues that incommensurability is always partial. Two paradigms may differ in many respects, but they share enough common ground — the demand for empirical adequacy, for logical consistency, for explanatory power — to make comparison possible. Moreover, the very fact that scientists can recognize that a problem is unsolved within their own paradigm shows that they can see beyond it. The scientist who says “my paradigm cannot explain this anomaly” has already acknowledged a standard that transcends the paradigm. Paradigm change is not a mystery; it is the normal process of learning from criticism.
The clarification is that Popper’s account of scientific revolutions is an extension of his theory of the growth of knowledge. Science grows by trial and error, by conjecture and refutation. Paradigms are large-scale conjectures. They are tested by their ability to solve problems, to survive criticism, and to generate new research. When a paradigm fails these tests, it is replaced by a better one — not by a perfect one, but by one that is a closer approximation to the truth. The rationality of this process is not guaranteed by any algorithm or decision procedure; it is guaranteed by the willingness of participants to engage in critical discussion. The open society of science is the model for the open society of politics. In both cases, progress depends not on agreement but on criticism, not on authority but on fallibility, not on certainty but on learning.
