Reasoning by Analogy

We reason by analogy constantly. A new situation resembles one we already understand, so we expect it to behave the same way. Doctors reason from similar past cases, judges from prior rulings, engineers from tested designs, and each of us from yesterday's experience to tomorrow's. Analogy is how we stretch hard-won knowledge into unfamiliar territory before we have direct evidence of our own.

The same move can quietly mislead. Two things can look alike on the surface and differ exactly where it counts, so a comparison that feels convincing carries a conclusion that does not hold. Reasoning by analogy well means learning which similarities do real work and which are only decoration.

The structure of an analogical argument

An analogy has a simple shape. Case A has features 1, 2, and 3, and also property P. Case B shares features 1, 2, and 3. Therefore B probably has P as well. The reasoning is inductive: the conclusion is made likely, never guaranteed. However close the match, the analogy could still fail, so its conclusion is always open to revision.

Relevant similarities and differences

Strength does not come from counting shared features. It comes from whether the shared features are causally or logically connected to the conclusion. A relevant similarity is one that bears on P; a relevant difference is one that would block it. Ten shared but irrelevant traits add nothing, while a single relevant difference can sink the whole argument.

False analogy

A false analogy leans on a resemblance that has nothing to do with the conclusion, or ignores a difference that matters. It borrows the feeling of a good comparison without the substance, and it is one of the oldest ways a persuasive line can be true-sounding and wrong.

"This restaurant will be excellent — it is run by the same owner and chef as the one downtown, and serves the same menu." The shared features here (owner, chef, menu) are causally tied to food quality, so the analogy is strong. Now compare a weaker version: "It will be excellent — it has the same green awning and is on a busy street." Those features resemble the first restaurant but have no bearing on the food. Same surface, different relevance, and only the first analogy earns its conclusion.

"Running a country is just like running a business, so a successful CEO will make a good president." The comparison is not empty — both involve budgets, staff, and hard decisions. But the relevant differences are large: a government does not seek profit, citizens are not customers who can be turned away, power is divided and checked, and leaders cannot simply fire the opposition. Because those differences bear directly on the conclusion, the shared traits cannot carry it. The analogy persuades only if you stop before weighing what is different.

In Natural Theology (1802), the English clergyman William Paley asked you to imagine crossing a heath and finding a watch. Its parts are arranged for a purpose, so you infer a maker. The eye and the bodies of animals show similar contrivance, Paley argued, so we should likewise infer a designer of nature. The whole argument rests on one analogy: living things resemble human-made machines.

David Hume had already probed that analogy decades earlier. In his Dialogues Concerning Natural Religion, published posthumously in 1779, the character Philo presses the design argument advanced by Cleanthes: the universe resembles a growing organism or a vegetable at least as much as a machine; we have no other universes to compare it with; and a single instance is thin ground for so large an inference. Paley wrote after Hume, yet did not answer him. The exchange is a lasting lesson in analogical reasoning, because the argument's force depends entirely on how relevant the likeness between organisms and artifacts really is.

• "More similarities always make a stronger analogy." Only relevant similarities count; irrelevant ones add nothing.
• "A good analogy proves its conclusion." Analogies support conclusions inductively; they never prove them.
• "Finding any difference destroys the analogy." Only a relevant difference weakens it; trivial differences do not.
• "Analogies are just rhetorical decoration." In law and science they do real inferential work and are argued over seriously.

Take an analogy you have heard recently and draw two columns: relevant similarities on one side, relevant differences on the other. Fill in each honestly, then decide which side carries more weight.

Think Like a Maester: An analogy is only as strong as the similarities that actually bear on the conclusion — count relevance, not resemblance.

1. What is the basic structure of an analogical argument?
2. What makes a similarity "relevant" to an analogy's conclusion?
3. What is a false analogy?
4. Why do analogical arguments yield probable rather than certain conclusions?
5. In the design argument, what similarity does the analogy rest on, and how did Hume challenge it?

Reasoning by analogy transfers a conclusion from a case you understand to a new one that resembles it: shared features make the new property likely, but only likely. What decides an analogy's strength is not how many features two cases share but whether the shared features are relevant to the conclusion, and whether some relevant difference breaks the link. A false analogy trades on resemblance that does no inferential work. Paley's watchmaker and Hume's earlier critique show the stakes in full: the same comparison can feel decisive or fall apart, depending entirely on how relevant the likeness really is. Weigh relevance, look hard for the difference that matters, and hold the conclusion as probable rather than proven.

• Logical Fallacies
• Constructing a Strong Argument