A false analogy is a faulty instance of the argument from analogy.
Argument from analogy
The process of analogical inference involves noting the shared properties of two or more things, and from this basis inferring that they also share some further property. The structure or form may be generalized like so:
- P and Q are similar in respect to properties a, b, and c.
- Object P has been observed to have further property x.
- Therefore, Q probably has property x also.
Several factors affect the strength of the argument from analogy:
- The relevance (positive or negative) of the known similarities to the similarity inferred in the conclusion.
- The degree of relevant similarity (or dissimilarity) between the two objects.
- The amount and variety of instances that form the basis of the analogy.
An argument from analogy is weakened if it is inadequate in any of the above respects. The term "false analogy" comes from the philosopher John Stuart Mill, who was one of the first individuals to engage in a detailed examination of analogical reasoning. One of Mill's examples involved an inference that some person is lazy from the observation that his or her sibling is lazy. According to Mill, sharing parents is not all that relevant to the property of laziness.
A basic example: "The model of the solar system is similar to that of an atom, with planets orbiting the sun like electrons orbiting the nucleus. Electrons can jump from orbit to orbit; so we should study ancient records for sightings of planets jumping from orbit to orbit."
Another example is:
Person A: "I think that people can have some affection for their cultural heritage."
Person B: "You're just like Hitler!"
In the above example, Person B has evaded a reasoned discussion by tarring Person A with an irrelevant association to an idea that Hitler used. Of course no one person is identical to another to the extent that their proposals can be disparaged by a mere reference to that other person. It is a form of ad hominem: Attacking the messenger, rather than the message. The above example is also an example of Reductio ad Hitlerum and, in an online context, invokes Godwin's law.