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.
It is necessary to take notice, that, as each of our senses has its
separate class of sensations, so each has its separate class of
ideas. We have ideas of Sight, ideas of Touch, ideas of Hearing,
ideas of Taste, and ideas of Smell.
1. By Sight, as we have sensations of red, yellow, blue, &c., and of
the innumerable modifications of them, so have we ideas of those
colours. We can think of those colours in the dark; that is, we have
a feeling or consciousness, which is not the same with the
sensation, but which we contemplate as a copy of the sensation, an
image of it; something more like it, than any thing else can be;
something which remains with us, after the sensation is gone, and
which, in the train of thought, we can use as its representative.