But now let us consider the cases in which we are not prepared to assert that the surface in question has changed perceptibly. The strange fact, from which the argument I mean is drawn, is that, in a very large number of such cases, it seems as if it were unmistakably true that the presented object, about which we are making our judgment when we talk of "This surface" at the later time, _is_ perceptibly different, from that about which we are making it when we talk of the surface I saw just now. If, at the later time, I am at a sufficiently greater distance from the surface, the presented object which corresponds to it at the time seems to be perceptibly smaller, than the one which corresponded to it before. If I am looking at it from a sufficiently oblique angle, the later presented object often seems to be perceptibly different in shape--a perceptibly flatter ellipse, for instance. If I am looking at it, with blue spectacles on, when formerly I had none, the later presented object seems to be perceptibly different in colour from the earlier one. If I am perceiving it by touch alone, whereas formerly I was perceiving it by sight alone, the later presented object seems to be perceptibly different from the earlier, in respect of the fact that it is not coloured at all, whereas the earlier was, and that, on the other hand, it has certain tactual qualities, which the earlier had not got. All this seems to be as plain as it can be, and yet it makes absolutely no difference to the fact that of the surface in question we are _not_ prepared to judge that it is perceptibly different from what it was. Sometimes, of course, where there seems to be no doubt that the later presented object is perceptibly different from the earlier, we may not notice that it is so. But even where we do notice the apparent difference, we do still continue to judge of the surface in question: This surface is not, so far as I can tell with certainty by perception, in any way different from what it was when I saw it or touched it just now; I am _not_ prepared to assert that it has changed perceptibly. It seems, therefore, to be absolutely impossible that the surface seen at the later time should be identical with the object presented then, and the surface seen at the earlier identical with the object presented then, for the simple reason that, whereas with regard to the later seen surface I am not prepared to judge that it is in any way perceptibly different from that seen earlier, it seems that with regard to the later sense-datum I cannot fail to judge that it _is_ perceptibly different from the earlier one: the fact that they are perceptibly different simply stares me in the face. It seems, in short, that when, in such a case, I judge: "This surface is not, so far as I can tell, perceptibly different from the one I saw just now," I cannot possibly be judging of the presented object "_This_ is not, so far as I can