QuestionsCan science explain everything?A theory is scientific if it can be refuted
Can science explain everything?True, but not everything

A theory is scientific if it can be refuted

Falsifiability

Karl Popper 1902 · 1994A theory is scientific if experience can refute it.
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What it claims

A theory is scientific only if a possible observation can refute it, and science advances by conjectures and refutations, not by piling up confirmations.

Why

A thousand white swans do not prove all swans are white, but one black swan refutes it: only possible refutation is decisive.

The same idea through history
David Hume1748Hume shows that no reasoning justifies the step from observed cases to a universal law. Popper does not try to get around this result: he accepts it and draws from it another picture of science, one that no longer needs induction.
Karl Popper1934In Logik der Forschung (1934) he proposes refutability as the criterion that separates science from what is not science. In 1963, in Conjectures and Refutations, he tells how the idea came to him in 1919, by comparing Einstein, Freud, Adler and Marx.
In short

"A theory is scientific if it can be refuted" is an argument by Karl Popper (Logik der Forschung, 1934; Conjectures and Refutations, 1963, 1934) on the question "Can science explain everything?" (answer: true, but not everything). Its claim: A theory is scientific only if a possible observation can refute it, and science advances by conjectures and refutations, not by piling up confirmations. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 4 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Logik der Forschung, 1934; Conjectures and Refutations, 1963
Valid with an implicit premise

How do we tell a scientific theory from one that only looks like it? In Vienna in 1919, the young Popper is struck by a fact. Marx, Freud and Adler explain everything: whatever happens, their followers see a confirmation. Einstein, by contrast, takes a risk: he announces that the light of stars will be bent near the Sun, and if the eclipse does not show it, his theory is false. Popper draws this idea from it. No number of observations can prove a universal law, but a single contrary observation can refute it. A theory is therefore scientific when it forbids something, that is, when a possible observation could contradict it. And science advances by proposing bold conjectures and then trying to refute them.

P1No number of particular observations logically justifies a universal law: even after seeing thousands of white swans, one cannot conclude that all swans are white.
P2A single contrary observation is enough to contradict a universal law logically: one black swan contradicts "all swans are white".
P3A theory is scientific only if experience can decide its fate. This is the basic thesis of empiricism: experience alone decides the truth or falsity of scientific statements.
P4 · implicit premiseFor a universal theory, experience can decide its fate in only two ways: by verifying it, or by refuting it with a contrary observation.
P5To test a theory seriously is to try to refute it. Confirmations are easy to find for nearly any theory: they count only if they come from a risky prediction, one that could have failed.
Therefore a theory is scientific only if a possible observation can refute it, and science advances by conjectures and refutations, not by piling up confirmations.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P4 the computer finds a case where verification is impossible, the theory is scientific, and yet it is not refutable: experience would then decide it in a way that neither verification nor refutation describes. P4 rules out that third means.
With P4 the conclusion follows: since verification is impossible (P1), the only way experience can decide is refutation (P4), so a scientific theory must be refutable (P3). Without P4 the computer finds a case where experience would decide by a third means. One point to note: the argument establishes that refutability is a necessary condition of science. Popper goes further when he calls refutability "the criterion of the scientific status" of a theory, but a refutable theory can be false, and a theory can be refutable without being good.
Analogy. A fortune-teller announces: "You will meet someone important." Whatever happens, he will be right. A weather forecaster announces: "It will rain in Paris tomorrow at noon." He may be wrong, and that is what makes his prediction serious. For Popper, science is like the forecaster, not like the fortune-teller.

Objections

4

Each objection is a conversation: click to see who replies and where the debate stands.

Pierre Duhem 1906 · attacks premise 2

The physicist can never submit a single hypothesis to the test of experiment, only a whole group of hypotheses. When experiment contradicts the prediction, it teaches him that at least one of the hypotheses is unacceptable, but it does not tell him which one. A contrary observation therefore never refutes a law by itself.

Popper replies
Popper 1963 · Popper's reply
He admits that often only a large chunk of a theoretical system can be tested, sometimes the whole system, and that in such cases deciding which element is responsible for the refutation is sheer guesswork. He says he has stressed this for a long time, himself also with reference to Duhem.
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Imre Lakatos 1970 · Lakatos's counter-reply
In the naive version of falsifiability, one contrary observation is enough to reject a theory. Lakatos finds that untenable. In the more refined version he attributes to Popper, no experiment alone can lead to falsification: there is no falsification before a better theory emerges.
The debate goes on (our summary) The disagreement is smaller than it looks. Duhem and Popper both admit that logic alone does not say which hypothesis to give up. Popper concludes that this must be settled by methodological rules, and that science is a game of conjectures. Lakatos concludes that the refutability of a single theory is not enough: series of theories must be judged (see the last objection).
Willard Van Orman Quine 1951 · attacks premise 2

Our statements about the external world do not face the tribunal of sense experience one by one: they face it together, as a corporate body. Quine thus extends the thesis of Duhem, whom he cites, to the whole of our knowledge.

Popper replies
Popper 1963 · Popper's reply
He answers that the holistic argument goes much too far. In quite a few cases one can find which hypothesis is responsible for the refutation, that is, which part of the system was necessary to derive the refuted prediction.
Thomas Kuhn 1962 · attacks premise 5

Scientists do not reject a theory at the first observation that contradicts it. In "normal science" they treat such anomalies as problems to be solved within the paradigm, and change paradigm only in times of crisis. Trying to refute one's own theory is therefore not what the researcher usually does (see Kuhn's argument on paradigms, in preparation).

Popper replies
Popper 1970 · Popper's reply
He writes that "normal" science in Kuhn's sense exists, and that he is indebted to Kuhn for pointing it out. But he regards it as a danger to science, whereas Kuhn apparently does not.
The debate goes on (our summary) Popper and Kuhn agree on the facts (normal science exists) and differ on its value: for Popper it is a danger, for Kuhn the normal condition of progress. The question of what scientists actually do, and what they ought to do, remains open.
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Imre Lakatos 1970 · attacks the conclusion

Two positions must be distinguished in Popper. "Naive" falsificationism (one contrary observation is enough to reject) is untenable, and Kuhn is right to criticise it. A more refined position is defensible: one does not judge an isolated theory but a series of theories, which must produce new facts. There is falsification only if a better theory exists.

no reply
Popper did reply, but that text is not yet in our corpus.
The debate goes on (our summary) Lakatos presents his version as a defence of Popper against Kuhn. Whether this version is Popper's or his own remains to be settled. And in this version one can no longer say, as in the argument, that a single observation is enough in scientific practice: it is enough only for logic.
What today's voices say
Defend it
Christopher HitchensHe cites Popper: unfalsifiability in a theory is a test not of its strength but of its weakness.
Karl PopperHis own criterion: a theory is scientific if possible observations could contradict it. In 1989: "we try to find out its weaknesses", imagining circumstances in which the hypothesis will not work, then testing them.
Lawrence KraussScience progresses because we can be wrong, unlike religion's claim to be absolutely right.
John LennoxHe argues that 'science is the only way to truth' is not itself a scientific statement, so scientism refutes itself.
Sabine HossenfelderShe says a model should be made more complicated only if data requires it. Models piled on the standard model without such data 'are not scientific hypotheses'.