This view, therefore, that what we are judging of the ultimate subject of our judgment, when we judge "This is a so-and-so," is, in general, merely that the subject in question is a _part_ of a thing of the kind in question, can, I think, be most clearly discussed, by asking whether, in this case, this presented object can really be identical with this part of the surface of this inkstand. If it can't, then most certainly I am not judging of it that it is a part of the surface of an inkstand at all. For my judgment, whatever it is, is true. And yet, if this presented object is not identical with this part of the surface of this inkstand, it certainly is not a part of an inkstand at all; since there is no other part, either of this inkstand or of any other, with which it could possibly be supposed to be identical.
Can we, then, hold that this sense-datum really is identical with this part of the surface of this inkstand? That everything which is true of the one is true of the other?
An enormous number of very familiar arguments have been used by various philosophers, which, if they were sound, would show that we can not. Some of these arguments seem to me to be quite clearly not sound--all, for instance, which rest either on the assumption that this sense-datum can only exist so long as it is perceived, or on the assumption that it can only exist so long as it is perceived _by me._ Of others I suspect that they may have some force, though I am quite unable to see that they have any. Such, for instance, are all those which assume either that this sense-datum is a sensation or feeling of mine, in a sense which includes the assertion that it is dependent on my mind in the very same sense in which my perception of it obviously is so; or that it is causally dependent on my body in the sense in which my perception of it admittedly is so. But others do seem to me to have great force. I will, however, confine myself to trying to state one, which seems to me to have as much as any. It will be found that this one involves an assumption, which does seem to me to have great force, but which yet seems to me to be doubtful. So far as I know, all good arguments against the view that this sense-datum really is identical with this part of the surface of the inkstand, do involve this same assumption, and have no more force than it has. But in this, of course, I may be wrong. Perhaps some one will be able to point out an argument, which is obviously quite independent of it, and which yet has force.