_Popular Use._--There is a popular use of this term which requires further notice. We speak of the identity of a mountain, a river, a tree, or any like object in nature. It is the same mountain, we say, that we looked upon in childhood, the same tree under which we sat when a boy, the same river in which we bathed or fished in youth. Now there is a sense in which this is true and correct. There has been change of substance unquestionably, and therefore there is not _absolute_ identity; but there is, after all, _numerical_ sameness, and this is what we mean when we speak of the sameness or identity of the object. It constitutes a sufficient ground for such use of terms. You recognize the book, the mountain, the river, as one you have seen before. The tree that you pass in your morning walk you recognize as the very tree under which you sat ten years ago. Leaves have changed, bark and fibres have changed; branches are larger and more numerous; boughs, perhaps, have fallen by time and by tempest; it has changed as you have changed, it has grown old like yourself, with changing seasons; its verdure and foliage, like your hopes and plans, lie scattered around it, and yet it is _to you_ the same tree. How so? It is the same numerical unity. Of a thousand or ten thousand _similar_ trees, similar in species, in growth, and form, and adaptation of parts, in size, color, general appearance, etc., it is this individual one, and not some other of the same sort or species growing elsewhere, that you refer to. It is the same numerical unity and not some other one of the series. Still there must be continuity of existence in order to identity even in this popular sense of the term. Were the parts entirely changed and new ones substituted, as in the puzzle of the knife with several successive handles and blades, or the ship whose original timbers, planks, cordage, and entire substance, had, in course of time, by continued repairs, been removed and replaced by new; in such a case, we do not ordinarily speak or think of the object as being any longer the same.