Thus the controversy concerning identity is not merely a dispute of words.  For when we attribute identity, in an improper sense, to variable or interrupted objects, our mistake is not confined to the expression, but is commonly attended with a fiction, either of something invariable and uninterrupted, or of something mysterious and inexplicable, or at least with a propensity to such fictions.  What will suffice to prove this hypothesis to the satisfaction of every fair enquirer, is to shew from daily experience and observation, that the objects, which are variable or interrupted, and yet are supposed to continue the same, are such only as consist of a succession of parts, connected together by resemblance, contiguity, or causation.  For as such a succession answers evidently to our notion of diversity, it can only be by mistake we ascribe to it an identity; and as the relation of parts, which leads us into this mistake, is really nothing but a quality, which produces an association of ideas, and an easy transition of the imagination from one to another, it can only be from the resemblance, which this act of the mind bears to that, by which we contemplate one continued object, that the error arises.  Our chief business, then, must be to prove, that all objects, to which we ascribe identity, without observing their invariableness and uninterruptedness, are such as consist of a succession of related objects.

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.