The case is not so much different in brutes but that any one may hence see what makes an animal and continues it the same. Something we have like this in machines, and may serve to illustrate it. For example, what is a watch ? It is plain it is nothing but a fit organization or construction of parts to a certain end, which, when a sufficient force is added to it, it is capable to attain. If we would suppose this machine one continued body, all whose organized parts were repaired, increased, or diminished by a constant addition or separation of insensible parts, with one common life, we should have something very much like the body of an animal ; with this difference, That, in an animal the fitness of the organization, and the motion wherein life consists, begin together, the motion coming from within ; but in machines the force coming sensibly from without, is often away when the organ is in order, and well fitted to receive it.
JOHN LOCKE. Identity of Animals. § 5. In: Identity and Diversity. Chap. XXVII. In: Of Ideas. Book II. In: An Essay concerning Human Understanding. Oxford: Oxford University Press, 1975.