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.
