Analogy 

Analogies are very useful in writing -- as long as you understand their limitations.

What is an analogy?

An analogy is a comparison in whichdifferent items are compared point by point, usually with the idea of explaining something unknown by something known. Analogies are offered to provide insights, and can be very instructive. Analogies tend to suggest that existing similarities imply even more similarities.

Formal analogies are in this general form:

This is read as follows: "a is to b as c is to d". What that means in plainer English is that the relationship between "a" and "b" is similar somehow to the relationship between "c" and "d."

Here are more specific examples:
1. shoe is to foot as tire is to wheel
2. followers are to a leader as planets are to a sun
3. shells were to ancient cultures as dollar bills are to modern culture

In each of these examples, there are parallels between the first to terms and the last two. In the first example, a shoe has the same relation to a foot that a tire has to a wheel. Followers, by the same logic, are similar to planents, and shells functioned in some ancient cultures as printed money functions in our culture today.

Although analogies are helpful in pointing out relationships that may not at first be visible, they have their limitations. You often hear it said that an analogy "breaks down." That means that it is only suggestive and does not follow in every detail.

In the first example above, for instance, there is only limited similarity between a shoe and a tire. I don't know if a tire "protects" a wheel, as a shoe protects a foot. Shoes are not paritcularly round. And you don't usually have to "air up" shoes. On the other hand, they can both wear out in similar ways and both are usually made of some pretty durable stuff.

Here's an example that may not be so familiar to the ordinary reader. If I am trying to explain certain features of cyberspace to readers not familiar with it, I might point out that on the Internet or the World Wide Web, the URL ("Uniform Resource Locator," that long "address" usually beginning like this, "http://www......") is analogous (or very similar) to a name of an ordinary file on your computer.

Here is the formal analogy:

Indeed, this analogy shows how a computer file and a WWW document name are very similar. What's even more interesting in this analogy is that it may be even MORE useful in showing the nature of the WWW: it's really just one huge computer. And because of the way files names are constructed, every document "address" is IN FACT just a file name on the enormous "worldwide" computer called the WWW.

The limitations of analogies have been suggested above: they break down or don't hold except in narrow ways. Another way of saying the same thing is that analogies don't prove anything. As said above, they are merely useful in helping people see similarities not otherwise apparent.