10 1. When we speak of them as composed of the same or different simple ideas;
2. When we speak of them as antecedent and consequent.
Whatever it may be necessary to remark, respecting relative terms, will occur in the consideration of these several cases.
I. 1. We speak of two sensations, as Same or Different, Like or Unlike.
These words are Relatives of the double signification; each individual of the pair has the same name. When we say that sensation A is the “same” with 11 sensation B, we mean that B also is the “same” with A; “different,” “like,” and “unlike,” have the same double application.
Another ambiguity needs to be noted in the word “same.” When there are two things, they are not the same thing; for “same,” in the strict sense of the word, means one thing, and that only. Here it means a great degree of likeness, a sense in which, with respect to sensations and ideas, it is very frequently used.
Of two sensations, or two ideas, we, in truth, can only say, that they are like or unlike; or, that the one comes first, the other after it.
It is now necessary to attend very carefully to what happens, when we say that two sensations are like, or that they are unlike.
First of all, we have the two sensations. But what is it to have two sensations? It is merely to be conscious of a change. But to be conscious of a change in sensation, is sensation. It is an essential part of the process. Without it we should not be sentient beings. To have sensation, and not to be conscious of any change, is to have but one sensation continued. We have already seen that this is a state which seems incapable of being distinguished from that of having no sensation. At any rate, what we mean by a sentient being, is not a being with one unvaried sensation, but a being with sensations continually varied; the varying being a necessary part of the having more sensations than one; and the varying, and the being conscious of the variation, being not two things, but one and the same thing. Having two sensations, therefore, is not only having sensation, but the only 12 thing which can, in strictness, be called having sensation; and the having two, and knowing they are two, which are not two things, but one and the same thing, is not only sensation, and nothing else than sensation, but the only thing which can, in strictness, be called sensation. The having a new sensation, and knowing that it is new, are not two things, but one and the same thing.[4]
[4] The author is here endeavouring to express the most fundamental fact of the consciousness—the necessity of change, or transition from one state to another in order to our being conscious. He approaches very near to, without exactly touching, the inference that all consciousness, all sensation, all knowledge must be of doubles; the state passed from and the state passed to, are equally recognised by us. Opening the eyes to the light, for the first time, we know a contrast,—a present light, a past privation—but for the one we should not have known the other. Any single thing is unknowable by us; its relative opposite is a part of its very existence.