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.

5.  Identity of Vegetables.

We must therefore consider wherein an oak differs from a mass of matter, and that seems to me to be in this, that the one is only the cohesion of particles of matter any how united, the other such a disposition of them as constitutes the parts of an oak; and such an organization of those parts as is fit to receive and distribute nourishment, so as to continue and frame the wood, bark, and leaves, &c., of an oak, in which consists the vegetable life.  That being then one plant which has such an organization of parts in one coherent body, partaking of one common life, it continues to be the same plant as long as it partakes of the same life, though that life be communicated to new particles of matter vitally united to the living plant, in a like continued organization conformable to that sort of plants.  For this organization,