THE sensations which we have through the medium of the senses exist only by the presence of the object, and cease upon its absence; nothing being here meant by the presence of the object, but that position of it with respect to the organ, which is the antecedent of the sensation; or by its absence, but any other position.
It is a known part of our constitution, that when our sensations cease, by the absence of their objects, something remains. After I have seen the sun, and 52 by shutting my eyes see him no longer, I can still think of him. I have still a feeling, the consequence of the sensation, which, though I can distinguish it from the sensation, and treat it as not the sensation, but something different from the sensation, is yet more like the sensation, than anything else can be; so like, that I call it a copy, an image, of the sensation; sometimes, a representation, or trace, of the sensation.
Another name, by which we denote this trace, this copy, of the sensation, which remains after the sensation ceases, is IDEA. This is a very convenient name, and it is that by which the copies of the sensation thus described will be commonly denominated in the present work. The word IDEA, in this sense, will express no theory whatsoever; nothing but the bare fact, which is indisputable. We have two classes of feelings; one, that which exists when the object of sense is present; another, that which exists after the object of sense has ceased to be present. The one class of feelings I call SENSATIONS; the other class of feelings I call IDEAS.
It is an inconvenience, that the word IDEA is used with great latitude of meaning, both in ordinary, and in philosophical discourse; and it will not be always expedient that I should avoid using it in senses different from that which I have now assigned. I trust, however, I shall in no case leave it doubtful, in what sense it is to be understood.
The term Sensation has a double meaning. It signifies not only an individual sensation; as when I say, I smell this rose, or I look at my hand: but it also signifies the general faculty of sensation; that is, 53 the complex notion of all the phenomena together, as a part of our nature.
The word Idea has only the meaning which corresponds to the first of those significations; it denotes an individual idea; and we have not a name for that complex notion which embraces, as one whole, all the different phenomena to which the term Idea relates. As we say Sensation, we might say also, Ideation; it would be a very useful word; and there is no objection to it, except the pedantic habit of decrying a new term. Sensation would in that case be the general name for one part of our constitution, Ideation for another.
It is of great importance, before the learner proceeds any farther, that he should not only have an accurate conception of this part of his constitution; but should acquire, by repetition, by complete familiarity, a ready habit of marking those immediate copies of his sensations, and of distinguishing them from every other phenomenon of his mind.
It has been represented, that the sensations of sight and hearing leave the most vivid traces; in other words, that the ideas corresponding to those sensations, are clearer than others. But what is meant by clearer and more vivid in this case, is not very apparent.
If I have a very clear idea of the colour of the trumpet which I have seen, and a very clear idea of its sound which I have heard, I have no less clear ideas of its shape, and of its size; ideas of the sensations, neither of the eye, nor of the ear.
It is not easy, in a subject like this, to determine what degree of illustration is needful. To those who are in the habit of distinguishing their mental 54 phenomena, the subject will appear too simple to require illustration. To those who are new to this important operation, a greater number of illustrations would be useful, than I shall deem it advisable to present.