We may thus distinguish three classes of ideas, which we have occasion to name: 1, simple ideas, the copies of single sensations: 2, complex ideas, copied directly from sensations: 3, complex ideas, derived indeed from the senses, but put together in arbitrary combinations. The two former classes may be called Sensible, the last Mental Ideas.
With respect to ideas, of the first two classes, those which are the direct copies of our sensations, either singly, or in groups; it is of great importance to observe, and also to remember, that, for the most part, the words, which are employed as marks of the Sensations, are made to serve the further purpose of being marks also of the Ideas. The same word is at once the name of the sensations, and the ideas.
If any person were asked, whether the word BEING is the name of a Sensation, or of an Idea; he would immediately reply, that it is the name of an Idea. In like manner, if he were asked, whether the word ANIMAL is the mark of a cluster of Sensations, or of a cluster of Ideas; he would with equal readiness say, of a cluster of Ideas. But if we were to ask, whether the name Sheep is the name of a cluster of Sensations, or of a cluster of Ideas; he would probably say, that Sheep is the name of Sensations; in the same manner as rose, or apple. Yet, what is the difference? Only this, that ANIMAL is the more general name, and includes sheep along with other species; and that BEING is still more general, and includes animal along with vegetable, mineral, and other { 139} _genera_. If sheep, therefore, or stone, be a name of sensations, so is animal or being; and if animal, or being, be a name of ideas, so is sheep or stone a name of ideas. The fact is, they are all names of both. They are names of the Sensations, primarily; but are afterwards employed as names also of the Ideas or copies of those sensations.
It thus appears, that the names generally of what are called the objects of sense are equivocal; and whereas it would have been a security against confusion to have been provided with appropriate names, one, in each instance, for the Sensation, and one for the Idea, the same name has been made to serve as the mark for both. The term horse is not only made to stand for the sensations of sight, of hearing, of touch, and even of smell, which give me occasion for the use of the term horse; but it stands also for the ideas of those sensations, as often as I have occasion to speak of that cluster of ideas which compose my notion of a horse. The term tree denotes undoubtedly the Idea in my mind, when I mean to convey the idea tree into the mind of another man; but it also stands for the sensations whence I have derived my idea of a tree.