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
Have a quick question? Jump to the Q&A page
Jump to a section
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").
Worked example: the design argument
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. To evaluate it, ask whether the shared feature (apparent order) is relevant to the target (having a designer), and whether there are relevant disanalogies. David Hume, anticipating Paley, pressed exactly this: 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.
Related dot points
- distinguish inductive from deductive reasoning and evaluate inductive arguments for strength and cogency rather than validity
A focused QCE Unit 3 answer on inductive reasoning. Covers the difference between deduction and induction, why inductive arguments are assessed for strength and cogency rather than validity, the role of probability, and how added evidence can defeat an otherwise strong inference.
- evaluate inductive generalisations by assessing sample size, representativeness and the dangers of hasty generalisation and biased sampling
A focused QCE Unit 3 answer on inductive generalisation. Covers the structure of generalising from a sample to a population, the criteria of sufficient size and representativeness, the fallacies of hasty generalisation and biased sampling, and why anecdotes and self-selected samples mislead.
- 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.