Karl Popper · Philosophy
Popper’s criterion of demarcation - what separates science from pseudo-science is not that a theory can be confirmed, but that it can be <em>refuted</em>: a scientific theory sticks its neck out, forbids certain observations, and risks being proved wrong.
In Vienna after the First World War, the young Karl Popper was surrounded by exciting new theories that all claimed the mantle of science: Einstein’s relativity, Marx’s theory of history, Freud’s psychoanalysis, Alfred Adler’s individual psychology. Their admirers spoke of each as a triumph of the scientific spirit. But Popper felt a nagging difference between Einstein and the rest, and spent years working out what it was. His question was not ‘which of these theories is true?’ but the sharper ‘what makes a theory scientific in the first place?’ This is the problem of demarcation - drawing the line between empirical science and non-science (which may be metaphysics, myth, or pseudo-science). The reigning answer was that science is what can be confirmed by observation. Popper came to think this was exactly backwards, because the theories he most distrusted were the ones for which confirmations were easiest to find.
Popper’s model of a scientific theory was Einstein’s general relativity. It predicted something startling and precise: that light from distant stars would be bent by the sun’s gravity by a specific amount, so that stars near the sun’s edge would appear shifted from their true positions. This was a risky prediction. If Arthur Eddington’s 1919 eclipse expedition had measured no such deflection, or the wrong amount, Einstein’s theory would have been refuted. Einstein had staked his theory on an observation that could have killed it. That, for Popper, is what makes a theory admirable and scientific: not that the eclipse observations happened to confirm it, but that Einstein had risked a definite forecast which reality could have shattered. A confirmation is impressive only when it is the survival of a genuine attempt at refutation.
Behind the criterion lies a simple point of logic that Popper prized. No number of confirming instances can ever prove a universal law. You may observe a million white swans, but the statement ‘all swans are white’ is never established, because the next swan might be black. Yet a single black swan is enough to refute it conclusively. There is a deep asymmetry between verification and falsification: universal theories can never be verified by experience, but they can be decisively falsified by it. This is why Popper made falsifiability, not verifiability, the criterion of the empirical character of a theory. A statement that says something about the real world puts itself at risk of collision with the facts; a statement that no possible fact could contradict has, in the end, said nothing about the world at all.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that Popper’s line between science and non-science is falsifiability : a scientific theory forbids certain possible events and can be refuted by observation, while an unfalsifiable theory (which fits every conceivable outcome) tells us nothing about the world. Explain the criterion, and why Popper thought…
Leads to Xenophanes.
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