Analogical reasoning infers that because two things are similar in relevant respects, they are likely similar in a further respect. The strength of an analogy depends on: the number of similarities, the relevance of those similarities to the conclusion, the absence of relevant disanalogies, and the scope of the claim being made. Analogies are the primary tool of legal reasoning (precedent), much of moral philosophy, and scientific hypothesis generation. A disanalogy — a relevant difference between the compared cases — is the standard way to challenge an analogical argument.
Evaluate the standard analogies in moral philosophy (e.g., Thomson's violinist analogy for abortion rights): list similarities, then list disanalogies, and assess whether the disanalogies undermine the conclusion. Practice generating disanalogies yourself.
You already understand inductive reasoning: reasoning from specific observations to probable generalizations. Analogical reasoning is a close cousin, but instead of generalizing from many cases to a type, you reason from one case to another based on their similarity. The basic structure is: case A has properties P1, P2, P3 and also property P4; case B has P1, P2, P3; therefore B probably also has P4. The move is licensed not by repetition across many instances but by the strength of the resemblance between A and B.
The central skill is learning to evaluate analogical strength. Three factors matter most. First, quantity of relevant similarities: the more shared features, the stronger the analogy — but only relevant ones count. That two countries share a coastline is unlikely to be relevant to whether they will have similar economic policies. Second, absence of relevant disanalogies: a single important difference between the cases can undermine even a superficially strong analogy. The question is always whether the difference is *relevant to the conclusion*. Third, scope of the claim: a narrow conclusion ("B will probably exhibit P4 in similar circumstances") is easier to support analogically than a sweeping one ("B is fundamentally just like A in all respects").
Legal reasoning depends almost entirely on analogical reasoning in the form of precedent. When a court decides a new case by looking to prior rulings, it asks: is this case sufficiently similar to the precedent that the same rule should apply? Arguments in legal briefs are often exercises in showing why the present case resembles — or does not resemble — an earlier decision. Moral philosophy uses analogies the same way. Judith Jarvis Thomson's famous violinist argument for abortion rights invites you to imagine waking up connected to a famous violinist whose survival depends on remaining connected to your circulatory system for nine months. The analogy is designed to test your intuitions by stripping away emotionally loaded features of the original case. The correct response is to assess whether the relevant similarities hold — and to look carefully for relevant disanalogies.
A disanalogy — a relevant difference between the compared cases — is the standard tool for rebutting an analogical argument. Critics of Thomson's violinist argue that the cases differ in morally relevant ways: the origin of the dependency (voluntary conception vs. waking up connected to a stranger), the nature of the relationship, and so on. The analogy may still be illuminating even if the disanalogies are real — it can shift the terms of debate, expose hidden assumptions, and narrow the actual point of disagreement. This is why analogical reasoning in philosophy is rarely a knockdown move; it is more often a framing device that forces explicit engagement with what we actually believe about the features being compared.
The practical lesson is to be systematic when evaluating analogies. List the similarities; then list the differences; then ask, for each item on each list, whether it is *relevant to the conclusion*. An analogy that survives this analysis — where similarities are relevant and disanalogies are not — is genuinely strong. An analogy that fails it — where the apparent similarities are superficial and the disanalogies cut to the heart of the conclusion — is weak regardless of how intuitively compelling the comparison first appeared.
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