to the mass, or subtracted from it; tho' 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 'tis nothing but a continu'd survey of the same object.
There is a very remarkable circumstance, that attends this experiment; which is, that tho' the change of any considerable part in a mass of matter destroys the identity of the whole, yet we must measure the greatness part, not absolutely, but by its proportion to the whole. The addition or diminution of a mountain wou'd not be sutficient to produce a diversity in a planet; tho' the change of a very few inches wou'd 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 [perfect?] [imperfect] identity.
This may be confirm'd by another phænomenon. A change in any considerable part of a body destroys its identity; but 'tis remarkable, that where the change is produc'd 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 continu'd perception, it ascribes a continu'd existence and identity to the object.
