=Use of the Term.=--A further problem regarding the process of conception concerns the question of the significance of a name. When a person uses such a term as dog, whale, hepatica, guava, etc., to name a certain object, what is the exact sense, or meaning, in which the name is to be applied? A class name, when applied scientifically to an object, is evidently supposed to denote the presence in it of certain essential characteristics which belong to the class. It is clear, however, that the ordinary man rarely uses these names with any scientific precision. A man can point to an object and say that it is a horse, and yet be ignorant of many of the essential features of a horse. In such cases, therefore, the use of the name merely shows that the person considers the object to belong to a certain class, but is no guarantee that he is thinking of the essential qualities of the class. It might be said, therefore, that a class term is used for two somewhat different purposes, either to denote the object merely, or to signify scientifically the attributes possessed by the object. It is in the second respect that danger of error in reasoning arises. So far as a name represents the attributes of a class, it will signify for us just those attributes which we associate with that class. So long, therefore, as the word fish means to us an animal living in the water, we will include in the class the whale, which really does not belong to the class, and perhaps exclude from the class the flying fish, although it is scientifically a member of the class.
It has been noted that, when man discovers common characteristics in a number of objects, he tends on this basis to unite such objects into a class. It is to be noted in addition, however, that in the same manner he is also able, by examining the characteristics of a large class of objects, to divide these into smaller sub-classes. Although, for example, we may place all three-sided figures into one class and call them triangles, we are further able to divide these into three sub-classes owing to certain differences that may be noted among them. Thus an important fact regarding classification is that while a class may possess some common quality or qualities, yet its members may be further divided into sub-classes and each of these smaller classes distinguished from the others by points of difference. Owing to this fact, there are two important elements entering into a scientific knowledge of any class, first, to know of what larger class it forms a part, and secondly, to know what characteristics distinguish it from the other classes which go with it to make up this larger class. To know the class equilateral triangle, for instance, we must know, first, that it belongs to the larger class triangle, and secondly, that it differs from other classes of triangles by having its three sides equal. For this reason a person is able to know a class scientifically without knowing all of its common characteristics. For instance, the large class of objects known as words is subdivided into smaller classes known as parts of speech. Taking one of these classes, the verb, we find that all verbs agree in possessing at least three common characteristics, they have power to assert, to denote manner, and to express time. To distinguish the verb, however, it is necessary to note only that it is a word used to assert, since this is the only characteristic which distinguishes it from the other classes of words. When, therefore, we describe any class of objects by first naming the larger class to which it belongs, and then stating the characteristics which distinguish it from the other co-ordinate classes, we are said to give a definition of the class, or to define it. The statement, "A trimeter is a verse of three measures," is a definition because it gives, first, the larger class (verse) to which the trimeters belong, and secondly, the difference (of three measures) which distinguishes the trimeter from all other verses. The statement, "A binomial is an algebraic expression consisting of two terms," is a definition, because it gives, first, the larger class (algebraic expression) to which binomials belong, and secondly, the difference (consisting of two terms) which distinguishes binomials from other algebraic expressions.