The argument I mean involves considerations which are exceedingly familiar, so familiar that I am afraid every one may be sick of hearing them alluded to. But, in spite of this fact, it seems to me not quite easy to put it quite precisely, in a way which will distinguish it clearly from other arguments involving the same familiar considerations, but which do not seem to me to be equally cogent. I want, therefore, to try to put it with a degree of precision, which will prevent irrelevant objections from being made to it--objections which would, I think, be relevant against some of these other arguments, but are not, I think, relevant against it. The fact is that we all, exceedingly commonly, when, at each of two times, separated by a longer or shorter interval, we see a part of the surface of a material thing, in the sense in which I am now seeing this part of the surface of this inkstand, or when at one time we see such a surface and at another perceive one by touch, make, on the second occasion, the judgment "_This_ part of a surface is the _same_ part of the surface of the same thing, as that which I was seeing (or perceiving by touch) just now." How commonly we all do this can scarcely be exaggerated. I look at this inkstand, and then I look again, and on the second occasion I judge "This part of the surface of this inkstand is the same as, or at least contains a part which is the same as a part of, the part of its surface which I was seeing just now." Or I look at this finger and then I touch it, and I judge, on the second occasion, "This part of the surface of this finger is the same as one of those I was seeing just now." We all thus constantly identify a part of a surface of a material thing which we are perceiving at one time with a part which we _were_ perceiving at another.