I assume, then, that we observe, on the one hand, coloured patches of certain shapes and sizes, and their spatial relations to one another, together with all the other kinds of "contents," which we should usually be said to perceive "through the senses." And, on the other hand, we also sometimes observe our own perceptions of such "contents" and our thoughts. And these two kinds of "content" are different from one another: my perception of a red patch with gold letters on it, is not itself a red patch with gold letters on it; and hence, when I observe my perception of this patch, I observe something different from that which I observe when I merely perceive the patch. Either of these two kinds of "content"--either colours, moving or at rest, sounds, smells, and all the rest--or, on the other hand, my perceptions of these--either of these two kinds, or both, might conceivably, since both are observed, give grounds for a generalisation concerning what exists. But, as I have said, if observations are to give any ground for such a generalisation, it must be assumed that what Is observed _exists_ or is _real._ And since, as I have insisted, when I observe my _perception_ of a red patch with gold letters on it, I observe something different from what I observed when I merely observed a red patch with gold letters on it, it follows that to assume the existence of my perception of this red and gold is _not_ the same thing as to assume the existence of the red and gold itself. But what, it may be asked, do I mean by this property of "existence" or "reality," which may, it would seem, belong to every content, which I observe, or may again belong to none, or which may belong to some and not to others? What is this property which may belong to my perception of a movement, and yet not belong to the movement perceived, or which may again belong to the movement perceived and not to my perception of it, or which may again belong to both or to neither?