to effect an actual and practical separation between these two things, light and seeing. By taking this step, you put an end to your perception; but you do not put an end, you say, to the real objective light which excited it. The perception has vanished, but the light remains, a permanent existence outside of your dark chamber. Now here we must beware of dogmatising, that is, of speaking either affirmatively or negatively, about anything, without first of all having thought about it. Before we can be entitled to speak of what is, we must ascertain what we can think. When, therefore, you talk of light as an outward permanent existence, we neither affirm nor deny it to be so. We give no opinion at all upon the matter. All that we request and expect of both of us is, that we shall think it before we talk of it. But we shall find that, the moment we think this outward permanent existence, we are forced, by the most stringent law of our intelligence, to think sight along with it; and it is only by thinking these two in inseparable unity, that light can become a conceivability at all, or a comprehensible thought.
Perhaps you will here remind us that light exists in many inaccessible regions, where it is neither seen nor was ever thought of as seen. It may be so; we do not deny it. But we answer that, before this light can be spoken of, it must be thought; and that it cannot be thought unless it be thought of as seen, unless we think an ideal spectator of it; in other words, unless a subjective be inseparably added unto
