In order to this, suppose any mass of matter, of which the parts are contiguous and connected, to be placed before us; it is plain we must attribute a perfect identity to this mass, provided all the parts continue uninterruptedly and invariably the same, whatever motion or change of place we may observe either in the whole or in any of the parts. But supposing some very small or inconsiderable part to be added to the mass, or subtracted from it; though this absolutely destroys the identity of the whole, strictly speaking; yet as we seldom think so accurately, we scruple not to pronounce a mass of matter the same, where we find so trivial an alteration. The passage of the thought from the object before the change to the object after it, is so smooth and easy, that we scarce perceive the transition, and are apt to imagine, that it is nothing but a continued survey of the same object. 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, let 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. It will 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 o 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 it is 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 at no particular time perceives any interruption in its actions. From which continued perception, it ascribes a continued existence and identity to the object.