1. Overview
Popper rejected the 300-year-old vision of science as induction — the idea that theories are justified by accumulating confirming observations. David Hume had already shown induction has no logical justification.
Popper's radical move: we do not need justification, we need criticism. Knowledge grows by bold conjectures that stick their neck out, followed by ruthless attempts to refute them.
- Rejection of justificationism: No theory is ever proven true, probable, or justified.
- Critical Rationalism: Rationality is the willingness to have your ideas criticized and to learn from error.
- Evolutionary epistemology: From amoeba to Einstein — all knowledge advances by trial and error elimination.
The growth of knowledge depends entirely upon disagreement.— Conjectures and Refutations, 1963
Popper's philosophy is optimistic: we can get closer to truth without ever possessing certainty.
2. The Demarcation Problem
What separates genuine science from pseudoscience, metaphysics, or ideology? For the Vienna Circle, it was verification. For Popper, verification is impossible — and worse, misleading.
A theory that explains everything explains nothing.— Popper, on Marxism, psychoanalysis, and astrology
The Criterion: Falsifiability
A theory is scientific if and only if it:
- Forbids some observable state of affairs.
- Is testable in principle — there must be a possible observation that would refute it.
- Exposes itself to severe tests, risking failure.
It does not matter how many confirmations it has. It matters how easily it could be shown wrong.
3. Falsification
The Problem of Induction
No number of white swans proves "All swans are white." Hume's point stands logically.
But one black swan falsifies it decisively. This is the asymmetry between verification and falsification.
Observing a non-white swan contradicts the theory by modus tollens.
Method
Bold Conjectures → Severe Tests → Refutation → New Conjecture
- Corroboration ≠ Confirmation: Passing a test doesn't make a theory true, only shows it has survived so far.
- Verisimilitude: Science aims at truthlikeness — theories with greater content, that forbid more.
- Naïve vs Sophisticated: Popper recognized the Duhem-Quine problem (we test theory + auxiliaries). Immunizing a theory with ad hoc excuses makes it unscientific — even if logically possible.
Swan Lab
Observe swans in Europe. How confirmed is the universal claim?
Bold vs Ad Hoc
A planet's orbit deviates. What do you do?
Theory: Newton's mechanics predicts elliptical orbit.
4. Why Popper Said Marxism and Psychoanalysis Are Not Scientific
Popper's break came in Vienna, 1919. After the Hörlgasse shooting, where police killed unarmed young socialists, Popper—then a teenage Marxist—realized that Marxist theory demanded sacrifice for a supposed historical inevitability. He turned to the question of intellectual honesty.
Marxism
Early Marx made risky, falsifiable predictions: revolution first in most advanced industrial societies; increasing misery of the proletariat; disappearance of the middle class.
When these failed (revolutions in backward Russia, rising living standards), followers did not abandon the theory. They added immunizing stratagems:
- "Imperialism delays the crisis"
- "False consciousness explains workers' contentment"
- "Revolution can occur in underdeveloped countries via vanguard"
The theory became compatible with any outcome — unfalsifiable historicism, like astrology.
Psychoanalysis (Freud & Adler)
Adler's inferiority complex and Freud's unconscious both explained behavior post-hoc. Popper watched Adler interpret a case:
Whatever the patient did could be fitted to the theory. Agreement confirmed it; disagreement was resistance, which also confirmed it.— Popper, Conjectures and Refutations
Contrast: Einstein 1919
| Theory | Risky Prediction | Could be falsified? |
|---|---|---|
| Einstein's GR | Starlight bends 1.75" by sun during eclipse (Eddington 1919) | Yes — precise, forbids other values |
| Marxism (post-1920) | Revolution happens... eventually, somehow | No — reinterpretable |
| Freudian analysis | Slip means unconscious wish; denial means repression | No — wins either way |
| Astrology | "You will face a challenge" (vague) | No |
5. Historicism & The Poverty of Historicism
Historicism is the doctrine that history follows inexorable laws, allowing us to predict the future course of society (Plato, Hegel, Marx).
Popper's logical argument against it:
- The course of human history is strongly influenced by the growth of scientific knowledge.
- We cannot predict, by rational methods, the future growth of our own knowledge (to predict it would be to already have it).
- Therefore, we cannot predict the future course of history.
Utopian Social Engineering
Historicism leads to grand blueprints: remake society as a whole toward a determined end. Since the plan cannot be tested piecemeal, failure is blamed on enemies or saboteurs — leading to totalitarianism.
Piecemeal Social Engineering
Popper's alternative: democratic reform through small, reversible experiments. Test policies like scientific hypotheses. Learn from mistakes. Preserve institutions that allow criticism.
6. Critical Rationalism
Popper abandons foundationalism. There are no secure foundations — no sense-data, no self-evident axioms. All knowledge remains conjectural.
- Objective knowledge: Knowledge is not a mental state, but the content of theories.
- Criticism as method: Rationality = openness to refutation, not possession of proof.
The Three Worlds
World 1
Physical objects & states
(rocks, brains, computers)
World 2
Subjective mental states
(beliefs, pains, dreams)
World 3
Objective contents of thought
(theories, books, scientific problems, falsifiers)
Knowledge in the objective sense is knowledge without a knower; it is knowledge without a knowing subject.— Objective Knowledge, 1972
7. The Open Society & Politics
Written in exile during WWII, The Open Society and Its Enemies defends liberal democracy against Plato, Hegel, and Marx.
Key Ideas
- Open vs Closed: A closed society claims to possess ultimate truth; an open society institutionalizes criticism.
- Protection of institutions: Free press, independent courts, democratic elections allow peaceful removal of bad rulers without violence.
- Negative utilitarianism: Priority should be to minimize avoidable suffering, not maximize abstract happiness.
The Paradox of Tolerance
Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant... then the tolerant will be destroyed, and tolerance with them.— The Open Society, Vol 1, note 4
We must therefore claim the right not to tolerate intolerance that suppresses rational debate.
8. Interactive Lab
Demarcation Tester
Paste a theory. Does it forbid anything observable?
Falsification Simulator
Create a universal claim. One counterexample kills it logically.
Marxism Transformer — how falsifiability disappears
Psychoanalysis Game — immunization
Analyst: "You have an unconscious Oedipus complex."
9. Criticisms & Legacy
Critiques
- Duhem-Quine: We never test a theory alone, only whole networks. Popper replies: we make a methodological decision which part to blame.
- Kuhn: Normal science works within paradigms; falsification rarely happens instantly. Popper admired bold science but underestimated tradition.
- Lakatos: Proposed "sophisticated falsificationism" — research programmes with hard cores and protective belts.
- Feyerabend: "Anything goes" — historical science is messier than Popper's logic.
Influence
- Science policy: distinction between science and pseudoscience in courts (e.g., US Daubert standard)
- Medicine: clinical trials designed to falsify, not confirm (randomized controlled trials)
- Philosophy: critical thinking education, fallibilism
- David Deutsch: Popperian epistemology extended to quantum computation and explanation
The more a theory forbids, the more it tells us.— The Logic of Scientific Discovery
Footnotes & Key Passages
1. LScD §6: "In so far as a scientific statement speaks about reality, it must be falsifiable; and in so far as it is not falsifiable, it does not speak about reality."
2. Conjectures and Refutations, Ch.1: On demarcation and the 1919 Eddington expedition.
3. The Open Society Vol.2, Ch.24: Critique of historicism and piecemeal engineering.