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.