The fourth step in conception is that of classification or generalization, whereby we place individual things in a mental bundle or class, and then this bundle in company with other bundles into a higher class, and so on. Thus we group all the individual small birds having certain characteristics into a species, then several related species into a larger family, and this into a still larger, until finally we group all the bird families into the great family which we call "birds" and of which the simple term "bird" expresses the general concept.
Jevons says: "We classify things together whenever we observe that they are like each other in any respect, and therefore think of them together. In classifying a collection of objects, we do not merely put together into groups those which resemble each other, but we also divide each class into smaller ones in which the resemblance is more complete. Thus the class of white substances may be divided into those which are solid, and those which are fluid, so that we get the two minor classes of solid-white and fluid-white substances. It is desirable to have names by which to show that one class is contained in another, and, accordingly, we call the class which is divided into two or more smaller ones the genus, and the smaller ones into which it is divided, the species."
Every species is a small family of the individuals composing it, and at the same time is an individual species of the genus just above it; the genus, in turn, is a family of several species, and at the same time an individual genus in the greater family or genus above it.
The student may familiarize himself with the idea of generalization by considering himself as an individual, John Smith. John represents that unit of generalization. The next step is to combine John with the other Smiths of his immediate family. Then this family may be grouped with his near blood relations, and so on, until finally all the related Smiths, near and remote, are grouped together in a great Smith family.
Or, in the same way, the family group may be enlarged until it takes in all the white people in a county, then all the white people in the state, then all in the United States; then all the white races, then all the white and other light-skinned races, then all mankind. Then, if one is inclined, the process may be continued until it embraces every living creature from moneron to man. Reversing the process, living creatures may be divided and subdivided until all mankind is seen to stand as a class. Then the race of man may be divided into sub-races according to color; then the white race may be subdivided into Americans and non-Americans. Then the Americans may be divided into inhabitants of the several states, or into Indianans and non-Indianans; then into the inhabitants of the several counties of Indiana, and thus the Posey Countians are reached. Then the Posey County people are divided into Smiths and non-Smiths; then the Smith family into its constituent family groups, and then into the smaller families, and so on, until the classification reaches one particular John Smith, who at last is found to be an individual—in a class by himself. This is the story of the ascending and descending processes of generalization.
CHAPTER XXII.
Classes of Concepts.
IN the preceding chapter we have seen the process of conception—of the forming of concepts. The idea of a general class of things or qualities is a concept. Each concept contains the qualities which are common to all the individuals composing the class, but not those qualities which pertain only to the minor classes or the individuals. For instance, the concept of "bird" will necessarily include the common qualities of warm-bloodedness, featheredness, wingedness, oviparousness, and vertebratedness. But it will not include color, special shape, size, or special features or characteristics of the subfamilies or individuals composing the great class. The class comprises the individuals and subclasses composing it; the concept includes the general and common qualities which all in the class possess. A percept is the mental image of a particular thing; a concept is the mental idea of the general qualities of a class of things. A percept arises from the perception of a sensation; a concept is a purely mental, abstract creation, whose only existence is in the world of ideas and which has no corresponding individual object in the world of sense.
There are two general classes of concepts, namely: (1) concrete concepts, in which the common qualities of a class of things are combined into one conceptual idea, such as "bird," of which we have spoken; (2) abstract concepts, in which is combined the idea of some quality common to a number of things, such as "sweetness" or "redness." Jevons's well-known rule for terms is an aid in remembering this classification: "A concrete term is the name of a thing; an abstract term is the name of a quality of a thing."
It is a peculiar fact and rule of concrete concepts that (1) the larger the class of things embraced in a concept, the smaller are its general qualities; and (2) the larger the number of general qualities included in a concept, the smaller the number of individuals embraced by it. For instance, the term "bird" embraces a great number of individuals—all the birds that are in existence, in fact, but it has but few general qualities, as we have seen. On the contrary, the concept "stork" has a much larger number of general qualities, but embraces far fewer individuals. Finally, the individual is reached, and we find that it has more qualities than any class can have; but it is composed of the smallest possible number of individuals, one. The secret is this: No two individuals can have as many qualities in common as each has individually, unless they are precisely alike, which is impossible in nature.