But whatever precaution we may use in introducing the changes gradually, and making them proportion able to the whole, 'tis certain, that where the changes are at last observ'd to become considerable, we make a scruple of ascribing identity to such different objects. There is, however, another artifice, by which we may induce the imagination to advance a step farther; and that is, by producing a reference of the parts to each other, and a combination to some common end or purpose. A ship, of which a considerable part has been chang'd by frequent reparations, is still consider'd as the same; nor does the difference of the materials hinder us from ascribing an identity to it. The common end, in which the parts conspire, is the same under all their variations, and affords an easy transition of the imagination from one situation of the body to another.
But this is still more remarkable, when we add a sympathy of parts to their common end, and suppose that they bear to each other, the reciprocal relation of cause and effect in all their actions and operations. This is the case with all animals and vegetables; where not only the several parts have a reference to some general purpose, but also a mutual dependance on, and connexion with each other. The effect of so strong a relation is, that tho' every one must allow, that in a very few years both vegetables and animals endure a total change, yet we still attribute identity to them, while their form, size, and substance are entirely alter'd. An oak, that grows from a small plant to a large tree, is still the same oak; tho' there be not one particle of matter, or figure of its parts the same. An infant becomes a man, and is sometimes fat, sometimes lean, without any change in his identity.
We may also consider the two following phænomena, which are remarkable in their kind. The first is, that tho' we commonly be able to distinguish pretty exactly betwixt numerical and specific identity, yet it sometimes happens,
that we confound them, and in our thinking and reasoning