"While speaking of resemblance, it is necessary to take notice of an ambiguity of language, against which scarcely any one is sufficiently on his guard. Resemblance, when it exists in the highest degree of all, amounting to undistinguishableness, is often called identity, and the two similar things are said to be the same. I say often, not always; for we do not say that two visible objects, two persons, for instance, are the same, because they are so much alike that one might be mistaken for the other: but we constantly use this mode of expression when speaking of feelings; as when I say that the sight of any object gives me the _same_ sensation or emotion to-day that it did yesterday, or the _same_ which it gives to some other person. This is evidently an incorrect application of the word _same_; for the feeling which I had yesterday is gone, never to return; what I have to-day is another feeling, exactly like the former, perhaps, but distinct from it; and it is evident that two different persons cannot be experiencing the same feeling, in the sense in which we say that they are both sitting at the same table. By a similar ambiguity we say, that two persons are ill of the _same_ disease; that two persons hold the _same_ office; not in the sense in which we say that they are engaged in the same adventure, or sailing in the same ship, but in the sense that they fill offices exactly similar, though, perhaps, in distant places. Great confusion of ideas is often produced, and many fallacies engendered, in otherwise enlightened understandings, by not being sufficiently alive to the fact (in itself not always to be avoided), that they use the same name to express ideas so different as those of identity and undistinguishable resemblance."[60] [60] "A System of Logic," Book I, Chap. III, § 11, N. Y., 1882, p. 62. It will be seen that Mill here draws a line between sameness in sense first and the samenesses in which there is an element of duality. He also draws attention to the fact--a fact to which I have already referred--that successive mental elements, considered in themselves, are more likely to be confounded than material things, though these last may be quite as closely similar. Language shows how men overlook the duality of two similar feelings which differ only in time. They may speak of two similar objects as the same, as they frequently do, and yet they will not usually lose the sense of their twoness. They say _these_ objects are the same. But when they compare a feeling experienced to-day with one experienced yesterday, they say _this_ is the same feeling I had yesterday. There is nothing in the language used to indicate duality at all.