It has been said that the Lockian substance is really an “unknowable.”  No one pretends to have experience of it; it is revealed to no sense; it is, indeed, a name for a mere nothing, for when we abstract from a thing, in thought, every single quality, we find that there is left to us nothing whatever.

We cannot say that the substance, in this sense of the word, is the reality of which the qualities are appearances.  In Chapter V we saw just what we may legitimately mean by realities and appearances, and it was made clear that an unknowable of any sort cannot possibly be the reality to which this or that appearance is referred.  Appearances and realities are experiences which are observed to be related in certain ways.  That which is not open to observation at all, that of which we have, and can have, no experience, we have no reason to call the reality of anything.  We have, in truth, no reason to talk about it at all, for we know nothing whatever about it; and when we do talk about it, it is because we are laboring under a delusion.

This is equally true whether we are concerned with the substance of material things or with the substance of minds.  An “unknowable” is an “unknowable” in any case, and we may simply discard it.  We lose nothing by so doing, for one cannot lose what one has never had, and what, by hypothesis, one can never have.  The loss of a mere word should occasion us no regret.

Now, we have seen that we do not lose the world of real material things in rejecting the “Unknowable” (Chapter V).  The things are complexes of qualities, of physical phenomena; and the more we know about these, the more do we know about real things.

But we have also seen (Chapter IV) that physical phenomena are not the only phenomena of which we have experience.  We are conscious of mental phenomena as well, of the phenomena of the subjective order, of sensations and ideas.  Why not admit that these constitute the mind, as physical phenomena constitute the things which belong to the external world?

He who says this says no more than that the mind is known and is knowable.  It is what it is perceived to be; and the more we know of mental phenomena, the more do we know of the mind.  Shall we call the mind as thus known a substance?  That depends on the significance which we give to this word.  It is better, perhaps, to avoid it, for it is fatally easy to slip into the old use of the word, and then to say, as men have said, that we do not know the mind as it is, but only as it appears to us to be—­that we do not know the reality, but only its appearances.

And if we keep clearly before us the view of the mind which I am advocating, we shall find an easy way out of the difficulties that seem to confront us when we consider it as nonextended and immaterial.

Certain complexes of mental phenomena—­for example, the barber’s pole above alluded to—­certainly appear to be extended.  Are they really extended?  If I imagine a tree a hundred feet high, is it really a hundred feet high?  Has it any real size at all?