This ought to become clear when we reflect upon the illustration of the fire.  We have seen that one whole series of changes has been unhesitatingly described as a series of changes in my sensations.  Why was this?  Because it was observed to depend upon changes in the relations of my body, my senses (a certain group of experiences), to the bit of experience I call the fire.  Another series was described as a series of changes in the fire.  Why?  Because, the relation to my senses remaining unchanged, changes still took place, and had to be accounted for in other ways.

It is a matter of common knowledge that they can be accounted for in other ways.  This is not a discovery of the philosopher.  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.  It is as unpardonable to confound the two as it is to confound sensations with things imaginary.

We may not, therefore, say that “things” are groups of sensations.  We may, if we please, describe them as complexes of qualities.  And we may not say that the “things” we perceive are really “inside” of us and are merely “projected outside.”

What can “inside” and “outside” mean?  Only this.  We recognize in our experience two distinct orders, the objective order, the system of phenomena which constitutes the material world, and the subjective order, the order of things mental, to which belong sensations and “ideas.”  That is “outside” which belongs to the objective order.  The word has no other meaning when used in this connection.  That is “inside” which belongs to the subjective order, and is contrasted with the former.

If we deny that there is an objective order, an external world, and say that everything is “inside,” we lose our distinction, and even the word “inside” becomes meaningless.  It indicates no contrast.  When men fall into the error of talking in this way, what they do is to keep the external world and gain the distinction, and at the same time to deny the existence of the world which has furnished it.  In other words, they put the clerk into a telephone exchange, and then tell us that the exchange does not really exist.  He is inside—­of what?  He is inside of nothing.  Then, can he really be inside?