He can only invite us to think over the matter and see what the unlearned and the learned are doing at every moment.

Sometimes they are noticing that experiences change as they turn their heads or walk toward or away from objects; sometimes they abstract from this, and consider the series of changes that take place independently of this.
That bit of experience, that red glow, is not related only to my body.
Such experiences are related also to each other; they stand in a vast independent system of relations, which, as we have seen, the man of science can study without troubling himself to consider sensations at all.

This system is the external world--the external world as known or as knowable, the only external world that it means anything for us to talk about.

As having its place in this system, a bit of experience is not a sensation, but is a quality or aspect of a thing.
Sensations, then, to be sensations, must be bits of experience considered in their relation to some organ of sense.

They should never be confused with qualities of things, which are experiences in a different setting.