Analogy compares two things, which are alike in several respects, for the purpose of explaining or clarifying some unfamiliar or difficult idea or object by showing how the idea or object is similar to some familiar one. While simile and analogy often overlap, the simile is generally a more artistic likening, done briefly for effect and emphasis, while analogy serves the more practical end of explaining a thought process or a line of reasoning or the abstract in terms of the concrete, and may therefore be more extended.

• You may abuse a tragedy, though you cannot write one. You may scold a carpenter who has made you a bad table, though you cannot make a table. It is not your trade to make tables. --Samuel Johnson
• He that voluntarily continues ignorance is guilty of all the crimes which ignorance produces, as to him that should extinguish the tapers of a lighthouse might justly be imputed the calamities of shipwrecks. --Samuel Johnson
• . . . For answers successfully arrived at are solutions to difficulties previously discussed, and one cannot untie a knot if he is ignorant of it. --Aristotle

Notice in these examples that the analogy is used to establish the pattern of reasoning by using a familiar or less abstract argument which the reader can understand easily and probably agree with.

Some analogies simply offer an explanation for clarification rather than a substitute argument:

• Knowledge always desires increase: it is like fire, which must first be kindled by some external agent, but which will afterwards propagate itself. --Samuel Johnson
• The beginning of all evil temptations is inconstancy of mind, and too little trust in God. For as a ship without a guide is driven hither and thither with every storm, so an unstable man, that anon leaveth his good purpose in God, is diversely tempted. The fire proveth gold, and temptation proveth the righteous man. --Thomas a Kempis

When the matter is complex and the analogy particularly useful for explaining it, the analogy can be extended into a rather long, multiple-point comparison:

• The body is a unit, though it is made up of many parts; and though all its parts are many, they form one body. So it is with Christ. (And so forth, to the end of the chapter.] --l Cor. 12:12 (NIV)

The importance of simile and analogy for teaching and writing cannot be overemphasized. To impress this upon you better, I would like to step aside a moment and offer two persuasive quotations:

• The country parson is full of all knowledge. They say, it is an ill mason that refuseth any stone: and there is no knowledge, but, in a skilful hand, serves either positively as it is, or else to illustrate some other knowledge. He condescends even to the knowledge of tillage, and pastorage, and makes great use of them in teaching, because people by what they understand are best led to what they understand not. --George Herbert
• To illustrate one thing by its resemblance to another has been always the most popular and efficacious art of instruction. There is indeed no other method of teaching that of which anyone is ignorant but by means of something already known; and a mind so enlarged by contemplation and enquiry that it has always many objects within its view will seldom be long without some near and familiar image through which an easy transition may be made to truths more distant and obscure. --Samuel Johnson