(II)
I now pass to the question how sensibles are related to physical objects. And here I want to say, to begin with, that I feel extremely puzzled about the whole subject. I find it extremely difficult to distinguish clearly from one another the different considerations which ought to be distinguished; and all I can do is to raise, more or less vaguely, certain questions as to how certain particular sensibles are related to certain particular physical objects, and to give the reasons which seem to me to have most weight for answering these questions in one way rather than another. I feel that all that I can say is very tentative.
To begin with, I do not know how "physical object" is to be defined, and I shall not try to define it. I shall, instead, consider certain propositions, which everybody will admit to be propositions about physical objects, and which I shall assume that I know to be true. And the question I shall raise is as to how these propositions are to be interpreted—in what sense they are true; in considering which, we shall at the same time consider how they are related to certain sensibles.
I am looking at two coins, one of which is a half-crown, the other a florin. Both are lying on the ground; and they are situated obliquely to my line of sight, so that the visual sensibles which I directly apprehend in looking at them are visibly elliptical, and not even approximately circular. Moreover, the half-crown is so much farther from me than the florin that its visual sensible is visibly smaller than that of the florin.
In these circumstances I am going to assume that I know the following propositions to be true; and no one, I think, will deny that we can know such propositions to be true, though, as we shall see, extremely different views may be taken as to what they mean. I know (a) that, in the ordinary sense of the word "see" I am really seeing two coins; an assertion which includes, if it is not identical with, the assertion that the visual experiences, which consist in my direct apprehension of those two elliptical patches of colour, are sensations proper, and are not either hallucinations nor mere experiences of "images"; (b) that the upper sides of the coins are really approximately circular, and not merely elliptical like the visual sensibles; (c) that the coins have another side, and an inside, though I don't see it; (d) that the upper side of the half-crown is really larger than that of the florin, though its visual sensible is smaller than the visual sensible of the upper side of the florin: (e) that both coins continue to exist, even when I turn away my head or shut my eyes; but in saying this, I do not, of course, mean to say that there is absolutely no change in them; I daresay there must be some change, and I do not know how to define exactly what I do mean. But we can, I think, say at least this: viz., that propositions (h), (c), and (d) will still be true, although proposition (a) has ceased to be true.
Now all these propositions are, I think, typical propositions of the sort which we call propositions about physical objects; and the two coins themselves are physical objects, if anything is. My question is: In what sense are these propositions true?
And in considering this question, there are, I think, two principles which we can lay down as certain to begin with; though they do not carry us very far.
The one is (a) that the upper side of the coin, which I am said to see, is not simply identical with the visual sensible which I directly apprehend in seeing it. That this is so might be thought to follow absolutely from each of the two facts which I have called (b) and (d); but I am not quite sure that it does follow from either of these or from both together: for it seems to me just possible that the two sensibles in question, though not circular in my private space, may yet be circular in physical space; and similarly that though the sensible of the half-crown is smaller than that of the florin in my private space, it may be larger in physical space. But what I think it does follow from is the fact that another person may be seeing the upper side of the coin in exactly the same sense in which I am seeing it, and yet his sensible be certainly different from mine. From this it follows absolutely that the upper side of the coin cannot be identical with both sensibles, since they are not identical with one another. And though it does not follow absolutely that it may not be identical with one of the two, yet it does follow that we can get a case in which it is not identical with mine and I need only assume that the case I am taking is such a case.
From this it follows that we must distinguish that sense of the word "see" in which we can be said to "see" a physical object, from that sense of the word in which "see" means merely to directly apprehend a visual sensible. In a proposition of the form "I see A," where A is a name or description of some physical object, though, if this proposition is to be true, there must be some visual sensible, B, which I am directly apprehending, yet the proposition "I see A" is certainly not always, and probably never, identical in meaning with the proposition "I directly apprehend B." In asserting "I see A" we are asserting not only that we directly apprehend some sensible but also something else about this sensible—it may be only some proposition of the form, "and this sensible has certain other properties," or it may be some proposition of the form "and I know this sensible to have certain other properties." Indeed we have not only to distinguish that sense of the word "perceive" in which it is equivalent to "directly apprehend," from one sense in which we can be said to perceive a physical object; we have also to distinguish at least two different senses in which we can be said to perceive physical objects, different both from one another and from "directly apprehend." For it is obvious that though I should be said to be now seeing the half-crown, there is a narrower, and more proper, sense, in which I can only be said to see one side of it—not its lower side or its inside, and not therefore the whole half-crown.
The other principle, which we can lay down to start with is (β) that my knowledge of all the five propositions (a) to (e), is based, in the last resort, on experiences of mine consisting in the direct apprehension of sensibles and in the perception of relations between directly apprehended sensibles. It is based on these, in at least this sense, that I should never have known any of these propositions if I had never directly apprehended any sensibles nor perceived any relations between them.