General Terms Difficult. That general terms and propositions are harder to get hold of than concrete facts is readily apparent in the first reading of an author like Emerson. To a young person it means little. Yet when he puts in the place of the general terms some specific examples, and so verifies the statements, the general propositions have a mine of meaning, and “the sense of the author is as broad as the world.” This stanza from Lowell is but little suggestive to young readers:—
“Such earnest natures are the fiery pith,
The compact nucleus, round which systems grow!
Mass after mass becomes inspired therewith,
And whirls impregnate with the central glow.”[14]
Yet when Columbus and Luther and Garrison are mentioned as illustrations of the meaning, it becomes [90] world-wide in its application. Still in order to get at the thought, there is first the need of the specific and the concrete; afterward we pass to the general and the abstract.
As abstract ideas are harder to get hold of than concrete facts, so exposition has difficulties greater than those found in narration and description. It is not so hard to tell what belongs in a story; the events are all distinct. Nor is it so difficult to know what to include in a description; one can look and see. In exposition this is not so. In most minds ideas do not have distinct limits; the edges rather are indistinct. It is hard to tell where the idea stops. In writing of “The Uses of Coal,” it is easy to wander over an indistinct boundary and to take a survey of “The Origin of Coal.” Not only may one include what unquestionably should be excluded, but there is no definite guide to the arrangement of the materials, such as was found in narration. There a sequence of time was an almost infallible rule; here the writer must search carefully how to arrange hazy ideas in some effective form. As discourse comes to deal more with general ideas, the difficulties of writing increase; and the difficulties are not due to any new principles of structure which must be introduced. When one says that the material should be selected according to the familiar law of Unity, he has given the guiding principle. Yet the real difficulty is still before an author: it is to decide what stamp to put upon such elusive matter as ideas. They cannot be kept long enough in the twilight of consciousness to analyze them; and often ideas that have been marked “accepted” have, upon reëxamination, to be “rejected.” To examine ideas—the material used in this form of discourse—so thoroughly that they may be accurately, definitely known in their backward [91] relation and their bearing upon what follows, this is the seat of the difficulty in exposition.
Exposition may conveniently be classified into exposition of a term, or definition; and exposition of a proposition, which is generally suggested by the term exposition.
Definition. Definition of a word means giving its limits or boundaries. Of man it might be said that it is a living animal, having a strong bony skeleton; that this skeleton consists of a trunk from which extend four limbs, called arms and legs, and is surmounted by a bony cavity, called a skull; that the skeleton protects the vital organs, and is itself covered by a muscular tissue which moves the bones and gives a rounded beauty to their ugliness; that man has a highly developed nervous system, the centre of which is the brain placed in the skull. So a person might go on for pages, enumerating the attributes which, taken together, make up the general idea of man.
Exposition and Description distinguished. This sort of exposition is very near description; indeed, were the purpose different, it would be description. The purpose, however, is not to tell how an individual looks, but to place the object in a class. It is therefore not description, but exposition. Moreover, the method is different. In description those characteristics are given that distinguish the object from the rest of the class; while in exposition those qualities are selected which are common to all objects of its class.
Logical Definition. On account of the length of the definition by an enumeration of all the attributes, it is not frequently used except in long treatises. For it there has been substituted what is called a logical definition. Instead of naming all the characteristics of an object, a logical definition groups many attributes [92] under one general term, and then adds a quality which distinguishes the object from the others of the general class. Man has been defined as the “reasoning animal.” In this definition a large number of attributes have been gathered together in the general term “animal;” then man is separated from the whole class “animal” by the word “reasoning.” A logical definition consists, then, of two parts: the general term naming the genus, and the limiting term naming the distinguishing attribute called the differentia.
Genus and Differentia. Genus and differentia are found in every good definition. The genus should be a term more general than the term defined. “Man is a person who reasons” is a poor definition; because “person” is no more general than “man.” “A canine is a dog that is wild” is very bad, because “dog,” the general term in the definition, is less general than the word defined. However, to say that “a dog is a canine that has been domesticated,” is a definition in which the genus is more general than the term defined.
Next, the genus should be a term well understood. “Man is a mammal who reasons” is all right, in having a genus more general than the term defined, but the definition fails with many because “mammal” is not well understood. “Botany is that branch of biology which treats of plant life” has in it the same error. “Biology” is not so well understood as “botany,” though it is a more general term. In cases of this sort, the writer should go farther toward the more general until he finds a term perfectly clear to all. “Man is an animal that reasons,” “botany is the branch of science that treats of plant life,” would both be easily understood. The genus should be a term better understood than the term defined; and it should be a term more general than the term defined.