Not only is it obvious that all these qualities are in us, but, if you will make the attempt, you will find it quite impossible to conceive of “blueness,” “roundness,” and “hardness” as existing without reference to some such consciousness as our own.  It may seem strange to say that even the “singleness” of the marble is relative to us; but extremely simple experiments will show that such is veritably the case, and that our two most trustworthy senses may be made to contradict one another on this very point.  Hold the marble between the finger and thumb, and look at it in the ordinary way.  Sight and touch agree that it is single.  Now squint, and sight tells you that there are two marbles, while touch asserts that there is only one.  Next, return the eyes to their natural position, and, having crossed the forefinger and the middle finger, put the marble between their tips.  Then touch will declare that there are two marbles, while sight says that there is only one; and touch claims our belief, when we attend to it, just as imperatively as sight does.

But it may be said, the marble takes up a certain space which could not be occupied, at the same time, by anything else.  In other words, the marble has the primary quality of matter, extension.  Surely this quality must be in the thing, and not in our minds?  But the reply must still be; whatever may, or may not, exist in the thing, all that we can know of these qualities is a state of consciousness.  What we call extension is a consciousness of a relation between two, or more, affections of the sense of sight, or of touch.  And it is wholly inconceivable that what we call extension should exist independently of such consciousness as our own.  Whether, notwithstanding this inconceivability, it does so exist, or not, is a point on which I offer no opinion.

Thus, whatever our marble may be in itself, all that we can know of it is under the shape of a bundle of our own consciousnesses.

Nor is our knowledge of anything we know or feel more, or less, than a knowledge of states of consciousness.  And our whole life is made up of such states.  Some of these states we refer to a cause we call “self;” others to a cause or causes which may be comprehended under the title of “not-self.”  But neither of the existence of “self,” nor of that of “not-self,” have we, or can we by any possibility have, any such unquestionable and immediate certainty as we have of the states of consciousness which we consider to be their effects.  They are not immediately observed facts, but results of the application of the law of causation to those facts.  Strictly speaking, the existence of a “self” and of a “not-self” are hypotheses by which we account for the facts of consciousness.  They stand upon the same footing as the belief in the general trustworthiness of memory, and in the general constancy of the order of nature—­as hypothetical assumptions which cannot be proved, or known with that highest degree of certainty which is given by immediate consciousness; but which, nevertheless, are of the highest practical value, inasmuch as the conclusions logically drawn from them are always verified by experience.