Every thing which is regarded by the mind as a separate existence must have some peculiar quality, or action, or circumstance of time or place, to distinguish it from every other existence. Were there not something, either in the qualities or circumstances, which made each object in some respects peculiar, there would be no way to distinguish one thing from another.
A proper name is one which is used to recall the properties and circumstances which distinguish one individual existence from every other. Such is the word Mount Blanc, which recalls certain qualities and circumstances that distinguish one particular thing from all others, and the name Julius Cæsar, which recalls the character, qualities, and circumstances which distinguish one being from every other.
Some words, then, are used to recall the peculiar qualities and circumstances of individual existences, and are called proper names; other words are used to recall a combination of certain qualities and circumstances, which unitedly are an object of conception, but are not considered by the mind as belonging to any real particular existence. These last words are called general terms or common names.
A great variety of names may be applied to the same object of conception or perception, according to the number of qualities and circumstances which are abstracted by the mind. Thus an object may be called a thing, and, in this case, the simple circumstance of existence is what is recalled by the word. The same object may be called an animal, and then the qualities of existence and animal life are made the objects of conception. It can also be called a man, and then, in addition to the qualities recalled by the word animal, are recalled those qualities which distinguish man from all other animals. It can also be called a father, and then to the qualities recalled by the term man is added the circumstance of his relation to some other being. The same object can be called La Fayette, and then, to all the preceding qualities, would be added in our conceptions all those peculiar qualities and circumstances which distinguish the hero of France from all other existences.
The following will probably illustrate the mode by which the human mind first acquires the proper use of these general terms. The infant child learns to distinguish one existence from another probably long before he acquires the use of any names by which to designate them. We may suppose that a little dog is an inmate of his nursery, and that with the sight of this animal has often been associated the sound of the word dog. This is so often repeated, that, by the principle of association, the sight of the object and the sound of the word invariably recur together. He observes that this sound is used by those around him in order to direct his attention to the animal, and he himself soon uses the word to direct the attention of others in the same way.
But soon it happens that another animal is introduced into his apartment, which in many respects resembles the object he has learned to call a dog. To this new object he would apply the same term, but he finds that others use the sound cat in connection with the sight of this new animal. He soon learns the difference between the two objects, the particulars in which they agree, and those in which they differ. He afterward notices other animals of these species, and observes that some have the qualities to which the term dog is applied, and others those to which the term cat is applied.
He continues to notice animals of other kinds, and, after long experience in this way, he learns to apply names to designate a particular combination of qualities, and, whenever these qualities are found combined, he has a term ready to apply to them. He learns that some words are used to point out the peculiar qualities which distinguish one thing from all others, and, at the same time, other words are used which simply recall qualities, but do not designate any particular existence to which they belong. Thus the term boy he uses for the purpose of designating qualities without conceiving of any particular existence in which they are found, while the term Mary is used to designate the qualities and circumstances of the particular existence he finds as the companion of his sports.
All objects of our perceptions are arranged into classes, according to the peculiar combination of qualities which are recalled by the names employed to designate them. For example, all objects that have the qualities of existence and of animal life are arranged in one class, and are called animals. All those which have the qualities recalled by the term animal, and the additional qualities of wings and feathers, are arranged in another class called birds. All those objects which have the qualities included in the term bird, together with several additional qualities, are arranged in another class, and called eagles.
To these various classes the terms genera and species are applied. These terms imply a relation, or the comparison of one class with another, in reference to the number of qualities to be recalled by the terms employed. Thus the class bird is called a species of the class animal, because it includes all the qualities that are combined in the conception recalled by the word animal, and others in addition; but the class bird is called a genus in relation to the class eagle, because it contains only a part of the qualities which are recalled by the term eagle.
A genus may be defined as a class of things the name of which recalls fewer particulars than the name of another class or species with which it is compared. Bird is a genus when compared with the class eagle.