In the second place, include everything that does properly belong in the definition. If you define a bridge as a roadway over a stream, either resting on piers or hanging on cables strung over towers, you exclude pontoon bridges certainly, and all bridges across dry chasms, if not other kinds. Not until you include all varieties of things crossed and all the methods of support and the various materials used will your definition be sound and complete. This does not mean that you will have to make an endless list of all possible forms, but that you will make a comprehensive statement which will allow of being distributed over all the different forms and kinds of bridges.
In the third place, use simple and familiar diction. Since the first purpose of a definition is to explain, one that is obscure or difficult makes confusion worse confounded. The famous—or notorious—definition which Dr. Johnson made of so simple a thing as network, "anything reticulated or decussated at equal distances with interstices between the intersections," is worse than useless because it positively throws dust upon a comparatively easy matter to perceive—unless the reader take time out for meditation. Remember that the Gettysburg Address and many of Shakespeare's sonnets are largely in words of one syllable. And then do not be afraid that you will be understood; the fire is always presumably somewhat more uncomfortable than the frying-pan.
In the fourth place, do not use the term that you are defining, or any derivative of it. When college freshmen, in mortal combat with a quiz question, define a description as something that describes, they use words that profit them nothing. That a cow is a cow is fairly obvious. The temptation to make this mistake, which, in the intellectual world, occupies the relative space of the saucy old advice, "Chase yourself round the block!" occurs usually when a long definition is being written, in which the writer forgets to keep the horizon clear, and finally falls into the formula x is x. To avoid yielding to such temptation, you will do well, after a definition is complete, to phrase it in a single sentence which shall include both differentia and genus, and in which you can easily discover the evil formula x is x. Bardolph, in Shakespeare's King Henry IV, yields to the temptation—for which we are glad as to humor but not made wise as to meaning—when Shallow puts him to the test:
Shallow: Better accommodated! it is good; yea, indeed, it is: good phrases are surely, and ever were, very commendable. Accommodated! it comes of accommodo: very good; a good phrase.
Bardolph: Pardon me, sir; I have heard the word. Phrase call you it? by this good day, I know not the phrase; but I will maintain the word with my sword to be a soldier-like word, and a word of exceeding good command, by heaven. Accommodated; that is, when a man is, as they say, accommodated; or when a man is, being, whereby 'a may be thought to be accommodated; which is an excellent thing.
In the fifth place, be sure that you define, and do not merely "talk about" the subject. Any amount of however interesting comment that fails to accomplish the two necessities of definition, statement of the genus and the differentia, is futile; it is not definition. This temptation, like the former one, will be overcome if you take the trouble to phrase the actual material of your definition in one sentence that really includes both genus and differentia. As a minor suggestion, do not begin your definition with the words, "X is when" or "X is where," unless you are defining either a unit of time or a place—and even then you will do well to avoid these too frequently used phrases.
Finally, do not make your definition too mechanical, too much lacking in real life. Thinking of how you must deal with genus and differentia, you are liable to be overwhelmed with the grim duty of being logical, and to forget that you should also be human, that people read definitions, as other kinds of writing, in the double hope of information and pleasure. No real antagonism exists between logic of the strictest kind and pleasurable presentation, as is proved by the examples quoted during the course of this chapter and at the end. While you remember your subject, remember also your reader; then you will be unlikely to make a dull definition.
Methods of Defining
You may use various methods of defining. Sometimes you will choose only one, and sometimes you will combine. There is no special virtue in one method more than another except as sometimes one happens to be more useful for a given case, as we shall see. In selecting your method, then, select on the basis of practical workability for the effect that you desire to create, adhering to one or using several as seems most effective.