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.