Extended Analogy
Extended Analogy is a logical fallacy that occurs when two things that are each analogous to a third are incorrectly concluded to be analogous to each other. A common example is the ad Hitlerum fallacy, which associates someone with Hitler based on a benign similarity.
THE IDEA
Extended Analogy
Two things may resemble a third in different respects without resembling each other in the respect an argument requires. For example, sharing an ordinary preference with Hitler does not establish that someone shares his cruelty.
The general form is: A resembles C in one respect; B also resembles C; therefore A and B must share some further trait. The conclusion needs evidence that the relevant trait carries across the comparisons.
Example #1: The original dialogue is missing from this excerpt. The supplied explanation describes Carl associating Jennie with fruitarians because they share a view about using animals for food. That shared view does not establish that Jennie shares their alleged “craziness”.
Example #2: The complete argument and its premises are also missing here. The commentary refers to premise two (P2) and a conclusion (p3), but these cannot be assessed as a formal argument without their wording. Its broader point is that an earlier scientific error does not establish that evolution is false. Saying science “was wrong” about atomic structure supplies no evidence that the same kind of error applies to evolution.
Exception: An analogy can support an inference when there is evidence that the relevant connection holds across the cases.
Tip: Don’t call people crazy – leave that kind of psychological assessment for the licensed professionals. You can call them, “nutjobs”.
When to use it
In political discourse, comparing someone to Hitler because they support a certain policy.
In debates about science, comparing evolution to past scientific mistakes.
In personal discussions, accusing someone of being part of a 'cult' because they have a similar diet to a cult.
In business, using analogy between two companies to argue they will fail for the same reason.
FROM THEORY TO EVERYDAY LIFE
In practice
Dietary Choices
The excerpt describes Carl comparing Jennie with fruitarians because they share a view about using animals for food, then attributing the fruitarians’ alleged “craziness” to her. Agreement on a dietary question does not establish that further similarity. The original dialogue is not included.
Evolution and Atomic Structure
The source commentary refers to a premise (P2) and conclusion (p3) that are missing from the excerpt. Its general objection is that science “was wrong” about an earlier model of the atom does not, by analogy alone, establish that evolution is wrong. The relevant evidence and reasons for error would have to be examined.
A MOMENT TO REFLECT
Put it to work.
Think of a recent argument where someone used an extended analogy. How could you point out the fallacy?
How can you avoid committing the extended analogy fallacy in your own reasoning?
Why is it important to check whether the similarities between analogies are relevant?
CONTINUE THE CONVERSATION
Source texts
- Logically Fallacious - The Ultimate Collection of Over 300 Logical Fallacies