II. EXPLANATION OF SIMPLE TERMS: DEFINITION

The simplest form of exposition is that which con¬sists in merely defining a term, or telling explicitly what it is in other words. To be able to do this well is important, for often before we can deal with a term
in extended exposition, or in any kind of writing, it is necessary to explain to our readers just what. the term includes, no more and no less. This is not so easy as it may at first seem.
33. Elements of a Definition.—Suppose that, glanc¬ing through the dictionary, we come across a word that is new to us, "kilderkin.' The dictionary pro¬ceeds to define it as "a measure." That is good as far as it goes. We know now to what class of objects the kilderkin belongs, that it is not an article of wear¬ing apparel, nor a wild animal; the genus, as we say, of the definition has been given us. But what does the kilderkin measure,—length, capacity, power? And how much does it measure? The rest of the definition satisfies these questions: "a liquid measure of a gallons." The particulars which distinguish this from other kinds of measures are now given, or, as we express it, the difference (Logic, differentia) has been supplied, and the definition is complete.
Every good definition, then, contains these two elements, genus and difference; that is to say, a general term that includes the object in a class, and one or more limiting terms which distinguish it from others of that class. In selecting and expressing these ele¬ments, some cautions are further to be observed.
1. Genus: What to Include.—We must take care that the genus, which denotes the class to which the object belongs, includes in its meaning mare than the word itself. "A pool is a small puddle of water" is an obviously faulty definition, because the word
"puddle" conveys a narrower idea to us than does the word "pool," a puddle being always conceived of as small and muddy. We can reverse the defini¬tion, therefore, and say that "a puddle is a small pool of muddy water"; but if we wish to define "pool," we have to look for a wider genus—" a small body" of water, for instance.
We must not, however, go to the extreme and select a genus that includes/too/much. "A sleeve is something that covers the arm" is a definition that clearly fails because the genus, "something," is wholly indefinite. It might refer to a surgeon's bandage. "That part of a garment which covers the arm" is much better.
2.	Genus: How to Express.—We must not fall into a careless form of speech and omit ' mentioning the genus altogether by employing a loose "when" or "where"• or "for." Definitions like "A cup is for drinking out of," "Vertigo is when you are dizzy," may express the desired idea roughly, but they are not exact enough to satisfy a logical thinker, and they are very bad specimens of English.
3.	Difference: What to Select.—In choosing the dif¬ference of a definition we should be careful to select qualities that belong to no other objects in the genus, and to select, moreover, not merely accidental qual¬ities that may at certain times or in certain instances pertain to the object, but the qualities that distinguish it at all times from other objects of its class. Thus, if we say that "a puddle is a small pool of water
caused by rain," again we have a faulty definition, for, in the first place, ponds may be said to be caused by rain, and in the second place puddles may be caused by the garden hose. We have not- selected, for our specific difference, the characteristic, or characteristics, which separate puddles from all other sorts of pools. "Small" and "muddy" answer the requirements better.
Other qualities, such as brevity and simplicity of diction, are desirable in a good definition, but the most important have been indicated. It should be evident that a little practice in constructing definitions will prove a valuable aid both to clear thinking and to good writing.