IDENTITY, denotes that by which a thing is itself; and not any thing else ; in which sense, identity differs from simili tude as well as diversity. The idea of identity we owe to that power which the mind has of comparing the very being and existence of things,w hereby, considering any thing as existing at any certain time and place, and comparing it with itself as existing at any other time and place, we accordingly pronounce it the same, or different. Thus, when we see a man at any time and place, and compare him with himself, when we see him again at any other time or place, we pronounce him to be the,same we saw before.
To Understand identity aright,we ought to consider the essence and existence, and the ideas these words stand for ; it being one thing to be the same substance ; an other, the same man ; and a third, the same person. For, suppose an atom exist ing at a determined time and place, it is the same with itself, and will continue so to be at any other instant, as long as its existence continues ; and the same may be said of two or any number of atoms, whilst they continue together ; the mass will be the same ; but if one atom be ta ken away, it is not the same mass. In animated beings it is otherwise, for the identity does not depend on the cohesion of its constituent particles, any how unit ed in one mass ; but on such a disposition and organization of parts, as is fit to re ceive and distribute life and nourishment to the whole frame. Man, therefore, who bath such an organization of parts partak ing of one common life, continues to be the same man, though that life be commu nicated to new succeeding particles of matter vitally united to the same organ ized body ; and in this consists the iden tity of man, considered as an animal only.
But personal identity, or the sameness of an intelligent being, consists in a continu ed consciousness of its being a thinking being, endowed with reason and reflec tion, capable of pain or pleasure, happi ness or misery, that considers itself the same thing in different times and places. By this consciousness every one is to him self, what he calls self, without consider ing whether that self be continued in the same or divers substances ; and so far as this consciousness extends backward to any past action, or thought, so far extends the identity of that person, and makes it the object of reward and punishment.— Hence it follows, that if the consciousness went with the hand, or any other limb, when severed from the body, it would be the same self that was just before concern ed for the whole. And if it were possible for the same man to have a distinct in communicable consciousness at different times, he would, without doubt, at differ ent times make different persons ; which we see is th,e sense of mankind as to mad men, for human laws do not punish the madman for the sober man's actions, nor the sober man for what the madman did, thereby considering them as two persons.