OF IDENTITY AND DIVERSITY.
1. Wherein Identity consists.
ANOTHER occasion the mind often takes of comparing, is the very being of things, when, considering ANYTHING AS EXISTING AT ANY DETERMINED TIME AND PLACE, we compare it with ITSELF EXISTING AT ANOTHER TIME, and thereon form the ideas of IDENTITY and DIVERSITY. When we see anything to be in any place in any instant of time, we are sure (be it what it will) that it is that very thing, and not another which at that same time exists in another place, how like and undistinguishable soever it may be in all other respects: and in this consists IDENTITY, when the ideas it is attributed to vary not at all from what they were that moment wherein we consider their former existence, and to which we compare the present. For we never finding, nor conceiving it possible, that two things of the same kind should exist in the same place at the same time, we rightly conclude, that, whatever exists anywhere at any time, excludes all of the same kind, and is there itself alone. When therefore we demand whether anything be the SAME or no, it refers always to something that existed such a time in such a place, which it was certain, at that instant, was the same with itself, and no other. From whence it follows, that one thing cannot have two beginnings of existence, nor two things one beginning; it being impossible for two things of the same kind to be or exist in the same instant, in the very same place; or one and the same thing in different places. That, therefore, that had one beginning, is the same thing; and that which had a different beginning in time and place from that, is not the same, but diverse. That which has made the difficulty about this relation has been the little care and attention used in having precise notions of the things to which it is attributed.
2. Identity of Substances.
We have the ideas but of three sorts of substances: 1. GOD. 2. FINITE INTELLIGENCES. 3. BODIES.
First, GOD is without beginning, eternal, unalterable, and everywhere, and therefore concerning his identity there can be no doubt.
Secondly, FINITE SPIRITS having had each its determinated time and place of beginning to exist, the relation to that time and place will always determine to each of them its identity, as long as it exists.
Thirdly, The same will hold of every PARTICLE OF MATTER, to which no addition or subtraction of matter being made, it is the same. For, though these three sorts of substances, as we term them, do not exclude one another out of the same place, yet we cannot conceive but that they must necessarily each of them exclude any of the same kind out of the same place: or else the notions and names of identity and diversity would be in vain, and there could be no such distinctions of substances, or anything else one from another. For example: could two bodies be in the same place at the same time; then those two parcels of matter must be one and the same, take them great or little; nay, all bodies must be one and the same. For, by the same reason that two particles of matter may be in one place, all bodies may be in one place: which, when it can be supposed, takes away the distinction of identity and diversity of one and more, and renders it ridiculous. But it being a contradiction that two or more should be one, identity and diversity are relations and ways of comparing well founded, and of use to the understanding.
3. Identity of modes and relations.
All other things being but modes or relations ultimately terminated in substances, the identity and diversity of each particular existence of them too will be by the same way determined: only as to things whose existence is in succession, such as are the actions of finite beings, v. g. MOTION and THOUGHT, both which consist in a continued train of succession, concerning THEIR diversity there can be no question: because each perishing the moment it begins, they cannot exist in different times, or in different places, as permanent beings can at different times exist in distant places; and therefore no motion or thought, considered as at different times, can be the same, each part thereof having a different beginning of existence.
4. Principium Individuationis.
From what has been said, it is easy to discover what is so much inquired after, the PRINCIPIUM INDIVIDUATIONIS; and that, it is plain, is existence itself; which determines a being of any sort to a particular time and place, incommunicable to two beings of the same kind. This, though it seems easier to conceive in simple substances or modes; yet, when reflected on, is not more difficult in compound ones, if care be taken to what it is applied: v.g. let us suppose an atom, i.e. a continued body under one immutable superficies, existing in a determined time and place; it is evident, that, considered in any instant of its existence, it is in that instant the same with itself. For, being at that instant what it is, and nothing else, it is the same, and so must continue as long as its existence is continued; for so long it will be the same, and no other. In like manner, if two or more atoms be joined together into the same mass, every one of those atoms will be the same, by the foregoing rule: and whilst they exist united together, the mass, consisting of the same atoms, must be the same mass, or the same body, let the parts be ever so differently jumbled. But if one of these atoms be taken away, or one new one added, it is no longer the same mass or the same body. In the state of living creatures, their identity depends not on a mass of the same particles, but on something else. For in them the variation of great parcels of matter alters not the identity: an oak growing from a plant to a great tree, and then lopped, is still the same oak; and a colt grown up to a horse, sometimes fat, sometimes lean, is all the while the same horse: though, in both these cases, there may be a manifest change of the parts; so that truly they are not either of them the same masses of matter, though they be truly one of them the same oak, and the other the same horse. The reason whereof is, that, in these two cases — a MASS OF MATTER and a LIVING BODY — identity is not applied to the same thing.