Arguments by analogy rest on a comparison. Their logical structure is this:
(1) A and B are similar.
(2) A has a certain characteristic.
(3) B must have that characteristic too.
For example, William Paley’s argument from design suggests that a watch and the universe are similar (both display order and complexity), and therefore infers from the fact that watches are the product of intelligent design that the universe must be a product of intelligent design too.
An argument by analogy is only as strong as the comparison on which it rests. The weak analogy fallacy (or “false analogy”, or “questionable analogy”) is committed when the comparison is not strong enough.
The example of an argument by analogy given above is controversial, but is arguably an example of a weak analogy. Are the similarities in the kind and degree of order exhibited by watches and the universe sufficient to support an inference to a similarity in their origins?