Argument from analogy

Argument from analogy is a special type of inductive argument, whereby perceived similarities are used as a basis to infer some further similarity that has yet to be observed. Analogical reasoning is one of the most common methods by which human beings attempt to understand the world and make decisions. When a person has a bad experience with a product and decides not to buy anything further from the producer, this is often a case of analogical reasoning. It is also implicit in much of science; for instance, experiments on laboratory rats typically proceed on the basis that some physiological similarities between rats and humans entails some further similarity (e.g. possible reactions to a drug).

Structure

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:

Of course, the argument doesn't assert that the two things are identical, only that they are similar. The argument may provide us with good evidence for the conclusion, but the conclusion does not follow as a matter of logical necessity. Determining the strength of the argument requires that we take into consideration more than just the form: the content must also come under scrutiny.