In the simplest terms, analogies are comparisons. In more specific terms, analogies are cognitive processes that have been employed since antiquity in the fields of linguistics, rhetoric, mathematics, engineering, and law. Analogies can be used to create or strengthen arguments. They can also be used to increase understanding of one topic by comparing it to another. This is done by comparing an unfamiliar topic or idea with one that is quite familiar. By making an analogy, the unfamiliar topic or idea becomes easier to understand.
Many people are familiar with analogies from their use in academic testing. Some educators and testing companies will check a student's understanding of one subject by comparing it with another subject. Let's say, for example, that an English teacher was testing her students' knowledge about the characters in Shakespeare's Hamlet. The teacher may include the following type of analogy question on the test.
Bart Simpson : Marge Simpson :: Prince Hamlet : ___________
This question is asking what character has a relationship with Prince Hamlet that is similar to the relationship between Bart and Marge Simpson. The answer to this question would be "Queen Gertrude" because Gertrude and Hamlet, like Marge and Bart, are mother and son. While it is arguable that Hamlet and Gertrude have a much more complex relationship than Bart and Marge, the analogy here is a tool simply to test knowledge of relationships. This example shows a flaw within analogies; although they can be very useful cognitive tools, they can also be reductive.
Analogies are often used in legal cases that do not correspond to a set precedent. If a legal case includes issues that have never before been handled in a court of law, then there is no precedent to use while judging them. In these cases, the legal professionals involved often search for cases that are somehow similar. In so doing, it is possible to create analogies between the case that is being tried and the previous, similar cases.
Analogies are often used in literature to help a reader understand an idea or to more fully visualize a scene. Here is an example of the way that an analogy can be used in fiction:
At the end of the worst day yet that year, the boy crawled under his bed covers the way a scared hermit crab retreats into its shell.
The analogy here is that the boy is to his bed as a hermit crab is to its shell or boy : bed :: hermit crab : shell. Similes and metaphors are closely related to analogies.