There is a very remarkable circumstance that attends this experiment; which is, that though the change of any considerable part in a mass of matter destroys the identity of the whole, yet we must measure the greatness of the part, not absolutely, but by its _proportion_ to the whole. The addition or diminution of a mountain would not be sufficient to produce a diversity in a planet; though the change of a very few inches would be able to destroy the identity of some bodies. 'Twill be impossible to account for this, but by reflecting that objects operate upon the mind, and break or interrupt the continuity of its actions, not according to their real greatness, but according to their proportion to each other; and therefore, since this interruption makes an object cease to appear the same, it must be the uninterrupted progress of the thought which constitutes the imperfect identity. This may be confirmed by another phenomenon. A change in any considerable part of a body destroys its identity; but 'tis remarkable, that where the change is produced _gradually_ and _insensibly_, we are less apt to ascribe to it the same effect. The reason can plainly be no other, than that the mind, in following the successive changes of the body, feels an easy passage from the surveying its condition in one moment, to the viewing of it in another, and in no particular time perceives any interruption in its actions. From which continued perception, it ascribes a continued existence and identity to the object. But whatever precaution we may use in introducing the changes gradually, and making them proportionable to the whole, 'tis certain, that where the changes are at last observed 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 changed by frequent reparations, is still considered 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.