explain inference to the best explanation (abduction) and evaluate hypotheses using criteria such as simplicity, explanatory scope and coherence

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.

A worked example

You return home to find the floor wet, the window open and a fallen vase. Many hypotheses fit: a burglar, a freak gust, a pet. The hypothesis "the cat knocked the vase while chasing something through the open window" may explain all three observations most simply and coherently, so you infer it. You are not certain, but it is the best available explanation.

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.