Argument by analogy
from 350 BCE Aristotle treats it as paradeigma; every legal tradition develops it independently
Two things resemble each other in several respects, so what is true of one is probably true of the other — the reasoning that carries all case law and most first guesses.
- Active
- from 350 BCE
- Where
- Athens, and independently elsewhere
- Status
- In use
- Tradition
- Forms of inference
What it is
An analogical argument identifies a resemblance between two cases and transfers a further property from the known one to the unknown. Its strength depends entirely on whether the resemblances noted are the ones that matter for the property being transferred.
It is not a weak deduction. It is a different thing altogether, and treating it as an inferior version of proof misses that it is the only method available when the case is genuinely new.
How it works
Set the two cases side by side. List the relevant similarities and, honestly, the relevant differences. Ask whether the similarities bear on the property in question or merely accompany it. Transfer the property with confidence proportional to that answer.
The third step is the one that separates a good analogy from a decorative one. Two things can resemble each other in twenty respects, none of which has anything to do with the conclusion.
The form of the inference
- Case A has properties p, q, r and also s
- Case B has properties p, q and r
- Properties p, q and r are the ones that bear on s
- Therefore case B probably also has s
The argument
Proposition The proposed roundabout will reduce crashes at this intersection.
- The intersection at Bangalow had the same layout: two through roads meeting a side road at an oblique angle, similar traffic volumes, and the same pattern of right-turn collisions.
- A roundabout was installed there in 2019 and right-turn collisions fell by eighty per cent over five years.
- The features that caused the collisions — the oblique angle and the right-turn conflict — are the ones the roundabout removes, and they are present in both cases.
- The two differ in rainfall and in the proportion of heavy vehicles, neither of which bore on the collision type in question.
Therefore A roundabout here will probably produce a comparable reduction.
The fourth step is the honest one and it is what most analogies leave out. Naming the differences and showing they are irrelevant is the work; listing similarities is easy and proves little.
What it is good for
Case law, medicine, engineering and every situation where a new case has to be decided and no rule covers it. It is also how almost all learning by example works.
Where it fails
It fails through irrelevant similarity, and it fails invisibly. Nothing in the form of the argument distinguishes resemblances that bear on the conclusion from resemblances that merely exist, and a long list of the second kind is highly persuasive and worth nothing.
It also has no natural stopping point. Any two things resemble each other in some respect, so an analogy can always be constructed, and the question is never whether one can be found but whether it should be trusted.
Common misconceptions
That pointing out a disanalogy defeats it. Every analogy has disanalogies — the two cases are not the same case. The reply only works if the difference bears on the property being transferred.
Where it came from
Aristotle discusses it under paradeigma, argument from example. Its most systematic developments are legal rather than philosophical: the common law doctrine of precedent and the Islamic doctrine of qiyas are both formal treatments of exactly this inference.
What became of it
Never formalised satisfactorily, and still the central mode of reasoning in law and in clinical practice.
Lasting influence
Both the common law and Islamic jurisprudence are built on it, which makes it arguably the most consequential form of reasoning on this list in terms of decisions actually made by it.
Why it still matters
Total. Every precedent cited in a court today is an argument by analogy.
Put another way
Judging a house by the one next door. Sometimes shrewd, sometimes exactly wrong.
Texts
Edward Levi, An Introduction to Legal Reasoning
Short, and the clearest account anywhere of how analogical reasoning actually works when decisions depend on it.
Where to start
Read two contrasting judgments on the same point of law. The disagreement will be about which similarities matter, which is the whole method in view.
The surprising bit
Two legal traditions with no contact between them, the English common law and Islamic fiqh, independently built their entire method of extension on this one inference.
In their own words
Reasoning by example in the law is a three-step process: similarity is seen, the rule is announced, the rule is applied
Who held it
Edward Levi
Connections
- Relies on
- Induction
- Parallel to
- The method of cases
The verdict
The most widely used and least formally understood inference in practical life.