Another kind of inductive argument compares two things: they are alike in some ways, so they are probably alike in another. This is an argument from analogy.
Key idea
An analogy is strong when the two things share features that are relevant to the conclusion, and differ only in ways that are not.
Why it works
An analogy works because the shared features are a reason for the shared conclusion. A new bridge with the same design, steel and builders as an old, sturdy one will likely be sturdy, because design, steel and builders are what make a bridge strong. Its color is irrelevant, so a shared color adds nothing.
Worked example: Which fact weakens it most?
"Ava's tomato plants and Ben's grow in the same soil and get the same sun. Ava's did well with weekly watering, so Ben's will too." (a) Ben's garden has a red gate. (b) Ben's plants are in small pots that dry out fast.
- Ask what the conclusion needs: plants that use water the same way.
- (a) is a difference, but a gate has nothing to do with water.
- (b) is a difference that matters for watering.
Answer: (b) weakens it most.
Worked example: Count what matters
A dog and a cat are both furry pets with four legs. Dogs love fetching sticks. Does that make it likely the cat will?
- Fur and four legs have nothing to do with liking to fetch.
- Many shared features, none relevant.
Answer: No: the analogy is weak.
Common mistake
Counting similarities instead of weighing them. Ten irrelevant likenesses add nothing; one relevant difference can sink the argument. Ask: what does the conclusion depend on, and do the two cases share that?