Let us follow out step by step the line of thought which this view makes its own. It is assumed that something happens outside of me in space; this produces an effect upon my sense-organs; and my nervous system conducts the impression thus made to my brain. There another occurrence is brought about. I experience the sensation “red.” Now follows the assertion: therefore the sensation “red” is not outside, not external to me; it is _in_ me. All our sensations are merely symbols or signs of external occurrences of whose real quality we know nothing. We live and move in our sensations and know nothing of their origin. In the spirit of this line of thought, it would thus be possible to assert that if we had no eyes, colour would not exist; for then there would be nothing to translate this, to us, wholly unknown external happening into the sensation “red.” For many people this line of thought possesses a curious attraction; but nevertheless it originates in a complete misconception of the facts under consideration. (Were it not that many of the present day scientists and philosophers are blinded even to absurdity by this line of thought, one would need to say less about it. But, as a matter of fact, this blindness has ruined in many respects the thinking of the present day.) In truth, since man is but one object or thing among other things, it naturally follows that if he is to have any experience of them at all, they must make an impression upon him somehow or other. Something that happens outside the man must cause something to happen within him, if in his visual field the sensation “red” is to make its appearance. The whole question turns upon this: What is without? what within? Outside of him something happens in space and time. But within there is undoubtedly a similar occurrence. For in the eye there occurs such a process, which manifests itself to the brain when I perceive the colour “red.” This process which goes on “inside” me, I cannot perceive directly, any more than I can directly perceive the wave motions “outside” which the physicist conceives of as answering to the colour “red.” But really it is only in this sense that I can speak of an “inside” and an “outside” at all. Only on the plane of sense-perception can the opposition between “outside” and “inside” hold good.