How does science test its theories, and what marks the boundary between science and non-science?
explain the hypothetico-deductive method and Popper's falsificationism, including the demarcation problem and the asymmetry of confirmation and refutation
A focused QCE Unit 3 answer on scientific method. Covers the hypothetico-deductive method, Karl Popper's falsificationism and demarcation criterion, the logical asymmetry between confirmation and refutation, the role of bold conjectures, and criticisms from Kuhn and the Duhem-Quine thesis.
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What this dot point is asking
QCAA wants you to explain how scientific theories are tested and what separates science from pseudoscience. The set framework is the hypothetico-deductive method and Karl Popper's falsificationism, including his solution to the demarcation problem. You need the logical structure, the key asymmetry between confirming and refuting a theory, and the main criticisms. This is the heart of the scientific reasoning strand.
The answer
The hypothetico-deductive method
The hypothetico-deductive (H-D) method describes science as follows: form a hypothesis, deduce an observable prediction from it, then test that prediction by observation or experiment. If the prediction comes true, the hypothesis is corroborated (supported, not proven). If it fails, the hypothesis is refuted. Crucially, the testing step is deductive: the prediction follows logically from the hypothesis, which lets a failed prediction strike back at the theory.
Popper and the asymmetry
Karl Popper, in The Logic of Scientific Discovery (1934), built on a logical asymmetry:
- No number of confirming instances can prove a universal law. "All swans are white" is not proven by a million white swans, because of Hume's problem of induction.
- But a single counter-instance can refute it. One black swan falsifies "all swans are white."
This asymmetry mirrors modus tollens: if the theory T entails prediction P, and P is false, then T is false. Confirmation is logically weak; refutation is logically decisive. Popper concluded that science does not work by verifying theories through induction but by attempting to falsify them. A good scientist proposes bold conjectures and tries hardest to refute them; theories that survive severe testing are corroborated but never certain.
The demarcation problem
The demarcation problem asks what distinguishes science from non-science or pseudoscience. Popper's answer: a theory is scientific only if it is falsifiable, that is, it forbids some observable outcome and so could in principle be proven wrong. Einstein's relativity made risky predictions (light bending by a precise amount) that could have failed, so it is scientific. Popper argued that some theories are framed so that no observation could ever count against them, making them unfalsifiable and, by his criterion, non-scientific however true they might feel.
Why falsifiability is a virtue
A theory that explains every possible outcome explains nothing, because it makes no risky predictions. Falsifiability forces a theory to stick its neck out. The more a theory forbids, the more it says and the more testable it is. This reframes the aim of science: not to accumulate confirmations but to make daring claims that survive serious attempts to break them.
Criticisms
- The Duhem-Quine thesis: hypotheses are never tested in isolation but only together with background assumptions. When a prediction fails, logic alone does not tell us which assumption to blame, so refutation is not as clean as Popper suggests; one can always save the core theory by adjusting an auxiliary assumption.
- Thomas Kuhn: in The Structure of Scientific Revolutions (1962), Kuhn argued that working scientists do not abandon a theory at the first anomaly; during normal science they treat anomalies as puzzles, and theories are overthrown only in occasional paradigm shifts. Real science looks less like relentless falsification than Popper claims.
- The role of confirmation: many argue science genuinely does accumulate inductive support, and that Popper cannot explain why we trust well-corroborated theories to keep working without smuggling induction back in.
Try this
Q1. Explain the logical asymmetry between confirming and refuting a universal law. [4 marks]
- Cue. No finite confirmations prove a universal claim; one counter-instance refutes it, as in modus tollens.
Q2. State Popper's demarcation criterion and give an example of a scientific theory by it. [3 marks]
- Cue. Falsifiability; relativity made the risky, testable prediction that light bends by a specific amount.
Q3. Explain how the Duhem-Quine thesis challenges falsificationism. [3 marks]
- Cue. A failed prediction tests a hypothesis plus background assumptions, so logic alone does not show which to reject.
Exam-style practice questions
Practice questions written in the style of QCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
QCAA 20226 marksExplain Popper's falsificationism and evaluate it as a solution to the demarcation problem.Show worked answer →
A 6 mark response sets out falsificationism, then judges it as a demarcation criterion.
Falsificationism. Building on the asymmetry that no confirmations prove a universal law but one counter-instance refutes it (mirroring modus tollens), Popper holds that science advances by bold conjectures subjected to severe attempts at refutation, with surviving theories corroborated but never proven.
Demarcation criterion. A theory is scientific only if it is falsifiable: it forbids some observable outcome and so could in principle be shown false. Relativity (the precise bending of light) is scientific; a theory compatible with every possible observation is not.
Evaluation. Strengths: it captures why risky, testable theories are scientific and dissolves reliance on inductive proof. Weaknesses: the Duhem-Quine thesis shows hypotheses are tested only with auxiliary assumptions, so a failed prediction does not cleanly falsify the core; Kuhn notes scientists keep theories despite anomalies. So falsifiability is a valuable but not decisive criterion.
Markers reward an accurate account of the asymmetry, the falsifiability criterion with an example, and a two-sided evaluation.
QCAA 20234 marksUsing an example, explain the hypothetico-deductive method and why its test step must be deductive.Show worked answer →
A 4 mark response gives the stages and explains the role of deduction.
Stages. Form a hypothesis, deduce an observable prediction from it, then test the prediction. Example: hypothesis "this metal expands when heated" yields the deduced prediction "this rod will lengthen when heated", which is then tested.
Why deductive. The prediction must follow logically (deductively) from the hypothesis so that, by modus tollens, a failed prediction strikes back at the hypothesis: if the rod does not lengthen, the hypothesis is refuted. If the link were merely probable, failure would not refute the theory.
Markers reward the three stages, an example, and the modus tollens point that deduction lets failure refute.