When does reasoning from one case to a similar case give a strong conclusion, and what makes an analogy break down?
analyse and evaluate arguments from analogy, assessing the relevance and number of similarities and the presence of relevant disanalogies
A focused QCE Unit 3 answer on analogical reasoning. Covers the structure of an argument from analogy, the criteria that make one strong (relevance, number and variety of similarities), how relevant disanalogies weaken it, and famous philosophical analogies such as Paley's watch and Thomson's violinist.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
QCAA wants you to analyse one of the most common inductive patterns: reasoning from one or more cases to a further case because they are alike. You must lay out the structure of an argument from analogy, state the criteria that make it strong or weak, and apply them to evaluate real philosophical analogies. This is core argument-analysis skill assessed across IA1, IA2 and the external exam.
The answer
The structure of an analogical argument
An argument from analogy infers that because two things share several features, they probably share a further feature. Its general form is:
- Object A has features f, g, h and target feature t.
- Object B has features f, g, h.
- Therefore object B probably also has feature t.
The argument is inductive: the shared features make the conclusion probable, not certain. Analogical reasoning is everywhere, from medical testing on animals to legal reasoning from precedent to design arguments for God.
Criteria for a strong analogy
Several factors raise or lower the strength of an analogical argument:
- Relevance of the similarities. This is the most important criterion. The shared features must be relevant to the target feature. Two cars sharing colour tells you nothing about reliability; sharing an engine type does.
- Number of relevant similarities. More relevant shared features generally strengthen the inference.
- Number and variety of the instances. Reasoning from many varied cases (not just one) to the conclusion is stronger.
- Relevant disanalogies. Differences that bear on the target feature weaken the argument. A single relevant disanalogy can defeat an otherwise strong analogy.
- Modesty of the conclusion. A weaker conclusion ("B probably has some t") is easier to support than a strong one ("B certainly has exactly t").
The design argument as an analogy
William Paley's watchmaker analogy argues: a watch shows order and purpose, and we infer it had a designer; the universe shows order and purpose; therefore the universe probably had a designer. David Hume, anticipating Paley, pressed exactly the relevance test: the universe is disanalogous to a watch in crucial ways (it grows, it is unique, we never observe universes being made), so the inference is weak. Charles Darwin later supplied a rival explanation of biological order without a designer.
Evaluating analogies in a response
To evaluate an argument from analogy in QCAA style: (1) identify the two things compared and the target feature; (2) list the similarities the argument relies on and ask whether they are relevant to that feature; (3) look for relevant disanalogies; (4) judge the overall strength. The decisive move is almost always relevance, not raw count: ten irrelevant similarities are worth less than one relevant one.
Where analogy sits in philosophy
Analogical reasoning drives many famous arguments. Judith Jarvis Thomson's violinist analogy argues about the morality of abortion by comparing pregnancy to being plugged into an ailing violinist. The argument from analogy for other minds infers that others have inner experiences because they behave as you do when you have experiences. In each case the philosophical fight is about whether the cases are relevantly similar, which is exactly the skill QCAA assesses.
Try this
Q1. State the general form of an argument from analogy. [2 marks]
- Cue. A and B share features f, g, h; A has t; so B probably has t.
Q2. Explain why relevance of similarities matters more than their number. [3 marks]
- Cue. Only similarities that bear on the target feature raise the probability of the conclusion; irrelevant ones do not.
Q3. Identify one relevant disanalogy in Paley's watchmaker argument. [3 marks]
- Cue. Unlike watches, the universe is unique and never observed being manufactured, so the inference to a designer weakens.
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 20227 marksAnalyse and evaluate the following argument from analogy: a country is like a household, so just as a household must not spend more than it earns, a government must not run a budget deficit. Assess the relevance of the similarities and identify any relevant disanalogies.Show worked answer →
A 7 mark response reconstructs the analogy, then evaluates it on relevance and disanalogy.
Reconstruction. A household and a government both receive income and spend; a household must avoid sustained overspending; therefore (by analogy) a government must avoid deficits.
Relevant similarities. Both have inflows and outflows and face a budget constraint, so the analogy is not empty.
Relevant disanalogies (the decisive move). A government, unlike a household, can issue its own currency, can tax, lives indefinitely so need not repay within a lifetime, and its spending can raise the national income that services the debt. These differences bear directly on the target feature (whether deficits are unsustainable), so they are relevant disanalogies that weaken the argument.
Verdict. The shared features are real but largely irrelevant to debt sustainability, while the disanalogies are highly relevant, so the analogical argument is weak.
Markers reward reconstruction, the focus on relevance over number, and at least one clearly relevant disanalogy.
QCAA 20235 marksExplain, with reference to Paley's watchmaker argument, why the relevance of similarities matters more than their number when evaluating an argument from analogy.Show worked answer →
A 5 mark response states the principle and applies it to Paley.
Principle. Analogical strength depends on whether the shared features bear on the target feature, not on how many features are shared. Ten irrelevant similarities add nothing; one relevant similarity can support the inference.
Application. Paley argues that a watch's order implies a designer, and the universe's order likewise implies a designer. Hume objects that the relevant similarity (apparent order produced by a mind) is exactly what is in dispute, while the universe is relevantly disanalogous to artefacts: it is unique, it grows organically, and we never observe universes being made. So piling up surface similarities does not help; only relevant ones would.
Markers reward the relevance principle and an accurate use of the Paley or Hume material.