When several hypotheses could explain the evidence, what justifies inferring the best one as true?
explain inference to the best explanation (abduction) and evaluate hypotheses using criteria such as simplicity, explanatory scope and coherence
A focused QCE Unit 3 answer on abductive reasoning. Covers the structure of inference to the best explanation, the criteria for ranking explanations (simplicity, scope, coherence, testability), the role of Ockham's razor, and the limits of abduction including underdetermination.
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What this dot point is asking
QCAA wants you to understand a third mode of reasoning alongside deduction and induction: abduction, or inference to the best explanation (IBE). You infer that the hypothesis which best explains the evidence is probably true. You need the structure, the criteria used to judge which explanation is best, and the limits of the inference. IBE is how detectives, doctors and scientists actually reason from evidence to cause.
The answer
The structure of abduction
An inference to the best explanation has the form:
- Some surprising fact E is observed.
- Hypothesis H, if true, would explain E.
- No rival hypothesis explains E as well as H.
- Therefore H is probably true.
This differs from deduction (which guarantees the conclusion) and from simple enumerative induction (which generalises from instances). IBE reasons backwards from an observation to its best cause or explanation. Charles Sanders Peirce named this pattern abduction.
Criteria for the best explanation
Calling an explanation "best" is not arbitrary; philosophers list criteria a good explanation tends to satisfy:
- Explanatory scope: it accounts for more of the evidence than rivals.
- Explanatory depth / power: it explains the evidence in detail, not just vaguely.
- Simplicity (Ockham's razor): it posits fewer entities or assumptions. William of Ockham's principle says do not multiply entities beyond necessity.
- Coherence: it fits with what we already know rather than requiring us to abandon well-established beliefs.
- Testability: it makes further predictions that could be checked.
- Conservatism: it disturbs our existing web of belief as little as possible while still doing the job.
An explanation that scores well across these criteria is preferable; trade-offs between them (a simpler theory with less scope, say) are where the hard judgement lies.
Abduction in science and philosophy
Much scientific reasoning is abductive. Darwin argued that natural selection was the best explanation of a huge range of biological facts (the fossil record, vestigial organs, biogeography), not that it was deduced from them. The case for unobservable entities such as atoms or dark matter is abductive: they are posited because they best explain observed phenomena. IBE is also used in philosophy itself, for example in arguing that scientific realism best explains the success of science.
Limits of abduction
IBE is inductive and so fallible. Three cautions:
- Underdetermination: more than one hypothesis may fit the evidence equally well, leaving no clear "best."
- The best of a bad lot: the best available explanation may still be poor if we have not thought of the true one; "best so far" is not "true."
- Bias in the criteria: judgements of simplicity and coherence can be subjective and theory-laden.
Despite these limits, abduction is indispensable; we could not reason from evidence to causes without it.
Try this
Q1. State the structure of an inference to the best explanation. [3 marks]
- Cue. Surprising fact E; H would explain E; no rival explains E as well; so H is probably true.
Q2. Name and explain two criteria for judging which explanation is best. [4 marks]
- Cue. For example explanatory scope (accounts for more evidence) and simplicity (fewer assumptions, Ockham's razor).
Q3. Explain the "best of a bad lot" objection. [3 marks]
- Cue. The best explanation considered may still be false if the true one was never among the alternatives.
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 inference to the best explanation, and use at least two criteria to evaluate which of two competing explanations of a surprising observation is better.Show worked answer →
A 6 mark response gives the structure of IBE and applies criteria to rank rivals.
Structure. A surprising fact E is observed; hypothesis H would explain E; no rival explains E as well; therefore H is probably true. The inference runs backwards from evidence to its best cause (abduction).
Worked comparison. Observation: a patient has a fever, rash and recent travel. H1: a common viral infection. H2: a rare tropical disease.
Apply criteria. Explanatory scope: if H1 leaves the travel history and specific rash unexplained while H2 accounts for all three, H2 has wider scope. Simplicity (Ockham's razor): H1 posits a more common cause, so it is simpler. Coherence and testability would then be weighed (a confirmatory blood test). The "best" explanation is the one that maximises scope and coherence at the least cost in assumptions.
Markers reward the IBE structure and the explicit use of at least two named criteria to rank the explanations.
QCAA 20234 marksExplain the "best of a bad lot" objection to inference to the best explanation and one implication it has for how we should use abduction.Show worked answer →
A 4 mark response states the objection and draws an implication.
Objection. IBE infers the best of the explanations we have actually considered, but if the true explanation was never among them, the "best" one selected may still be false. Calling it best ranks it only within a possibly poor field.
Implication. We should treat the conclusion of an abductive inference as provisional and actively widen the field of hypotheses considered, rather than treating "best so far" as "true". It also warns against premature confidence when only one or two explanations have been canvassed.
Markers reward an accurate statement of the objection and a sensible methodological implication.