Three years after the Essay was published, Locke's friend William Molyneux wrote from Ireland with several suggestions. Although he greatly admired Locke's achievment, Molyneux proposed recasting it as a scholastic textbook on logic and metaphysics, with a supplementary volume dealing more fully with human action and morality. Specifically, Molyneux invited his friend to "Insist more particularly and at Large on Æterna Veritates and the Principium Individuationis." [Corr. 1609] Locke and Molyneux clearly shared a conviction that the attribution of moral responsibility and the justice of moral sanctions depends upon the persistent identity of the moral agent. But in the Essay's first edition, Locke had pointed out two significant difficulties: First, of course, he denied that the personal identity of moral agents can be known innately. [Essay I iii 3-5] Secondly, he had argued that Cartesian dualism cannot adequately ground personal identity on the substantial identity of the soul, the body, or their composite. [Essay II i 11-12] Now Molyneux demanded that Locke provide an account of his own, and a few months later, he had prepared a draft of a new chapter (what would become II xxvii) for the second edition of the Essay. [Corr. 1655]
The basic notion of identity (and diversity) arises from the simple fact that no two things of the same kind can co-exist in the same place; extended through time, this entails that every individual must have a spatio-temporal history that is unique among others of its kind. [Essay II xxvii 1] Thus, Locke held,
Of course, we can make intuitive judgments of identity and diversity only if our ideas of the thing or things involved are clear, but the crucial insight of Locke's theory of identity is that it must be applied sortally, with respect to things belonging to a common kind. No matter what in particular happens to constitute the very existence of a thing as it is conceived under a sortal term and the complex idea it signifies, Locke supposed, the identity of that thing through time just is its continued existence as a thing of this sort. [Essay II xxvii 28-29]
From what has been said, 'tis easy to discover, what is so much enquired after, the principium Individuationis, and that 'tis plain is Existence it self, which determines a Being of any sort to a particular time and place incommunicable to two Beings of the same kind. [Essay II xxvii 3]
On Locke's view, then: God continuously exists unchanged in all places and at all times; each finite spirit begins to exist and continues to exist distinctly from every other thinking thing; particular bodies must each have their own unique spatio-temporal histories; non-substantial modes and relations typically persist as features of one or more substances; and even composite objects derive their identity from the collection of things that constitute them. Although any of these varieties of "thing" may coexist with any of the others, each uniquely occupies its own "space" within the sort of which it is a member. [Essay II xxvii 2-3]
Although living things similarly comprise a collection of material particles, Locke carefully noted, their persistence through time depends less upon the preservation of the same collection than upon the pattern of organization it exhibits at different times. The full-grown horse is the same animal as the colt of a few years hence, and the mighty oak is the same tree as the sapling of a century ago, even though the particular bits of matter each includes are distinct. What matters in such cases, according to Locke, is the continuous (vegetable or animal) "Life" of the thing, the organization of material parts in a way that creates and preserves its most characteristic functions in the world. [Essay II xxvii 3-5]
The identity of an individual human being ("Man") rests upon exactly the same foundation in Locke's theory. Human beings should be defined not by their rationality (else we be forced to call the rational Brazilian parrot a man) nor by their presumed annexation to an immaterial soul (else the same man may exist in different centuries) but rather as living animals of a particular species, with its characteristic structure and function. But then human identity is just animal identity: at any one time, there is a collection of material parts organized in a fashion suitable for the support of human life, and that life persists through the continuation of this pattern of organization even when its particular material constituents are successively annexed and removed. I am the same human being as my mother's first-born child, despite the obvious alterations of a half-century, because my "life"understood as an ongoing principle of organizationhas been continuous. [Essay II xxvii 6-8]
The person is something else entirely. Locke's account of the demonstrability of morality relied upon an abstract conception of the moral agent, or "the Moral Man," understood simply in terms of its functions in contemplating and performing actions"a corporeal rational Being." [Essay III xi 16] But that is precisely the definition Locke now provides for the notion of the "Self" or "Person:"
It is a Forensick Term appropriating Actions and their Merit; and so belongs only to intelligent Agents capable of a Law, and Happiness and Misery. [Essay II xxvii 26]But since the person is a different kind of thing, the criteria for the identity of a particular person through time will also be different from those that apply for substances, composites, and even living human beings.
Having defined the person in terms of its function rather than by reference to its underlying nature, Locke explicitly maintained that the identity of a conscious person is independent of the identity of whatever substance (or substances) happen to compose it at any time. [Essay II xxvii 9-10] This view has some clear advantages in the effort to provide a secure foundation for moral reasoning: a non-substantial account personal identity renders morally irrelevant all metaphysical disputes about human nature, since the continued existence of the same substancematerial or immaterialis neither necessary nor sufficient for that of the moral agent. [Essay II xxvii 24-25] In particular, Locke took great pains in showing that the Cartesian account of human nature, as an immaterial thinking substance existing in uneasy alliance with a differentiated portion of the material universe, is inadequate for the allocation of just punishment to the same moral agent who commits an immoral act. Several of Locke's notorious "puzzle cases" are intended precisely to undermine any attempt at a Cartesian explanation of moral accountability. [Essay II xxvii 10-14]
The other "puzzle cases" illustrate the independence of personal identity from the animal identity of any human organism. Although "same person" and "same human being" are sometimes used interchangeably in ordinary discourse, Locke noted, they are philosophically distinct. One and the same human beingwhile awake and while asleepcould participate in conscious lives that were as completely distinct from each other in the moral sense as are the distinct lives of identical twins. Even the legal tradition preserves this point by declining to punish a human being for actions performed while insane, since these crimes were, for all moral purposes, committed by a different person. [Essay II xxvii 15-20] A morally adequate position must somehow account for personal identity without reference to either an immaterial soul or a living body.
According to Locke, an appropriate account of personal identity must arise from careful analysis of the concept of the person. Since self-conscious awareness invariably accompanies all human thought, it alone can both distinguish the self from every other thinking thing in the present and preserve its identity through time.
Since only consciousness of pleasure and pain can support both desire for one's own welfare and accountability for one's own actions, it is by consciousness alone that the self appropriates those present or past actions for which it now or in the future justly deserves to be punished or rewarded. [Essay II xxvii 23, 26]
This being premised to find wherein personal Identity consists, we must consider what Person stands for; which, I think, is a thinking intelligent Being, that has reason and reflection, and can consider it self as it self, the same thinking thing in different times and places; which it does only by that consciousness, which is inseparable from thinking, and as it seems to me essential to it. [Essay II xxvii 9]
Consciousness is both a necessary and a sufficient condition for a morally vital sense of personal identity. It is necessary because no past thought or action was truly mine unless I now self-consciously appropriate it to myself; and from this it follows that I cannot now be justly punished as the agent who committed some past action unless I am conscious of having performed that action myself. [Essay II xxvii 18-20] It is sufficient because consciousness unites temporally distinct thoughts and actions from past, present, and future into a single person; since I can now harbor concern for the happiness or misery of the future self that would justly suffer punishment for my present transgressions, deliberation about those consequences are a relevent force in motivating my present conduct. [Essay II xxvii 15-18] It all comes down to this:
For as far as any intelligent Being can repeat the Idea of any past Action with the same consciousness it had of it at first, and with the same consciousness it has of any present Action; so far it is the same personal self. For it is by the consciousness it has of its present Thoughts and Actions, that it is self to it self now, and so will be the same self as far as the same consciousness can extend to Actions past or to come. [Essay II xxvii 10]
One difficulty with Locke's explanation is that the self-conscious awareness he holds to be constituitive of personal identity is, by its very nature, accessible only to the individual self. It follows that third-person judgments of personal identity are systematically liable to error. For the allocation of punishment and reward, human judicatures must rely upon the presumed association of conscious personal identity with that of a living human body. Although it regards this presumption as defeasible in cases of madness or somnambulism, the civil law rightly focusses upon what it can prove rather than upon what it can only suppose to the contrary. [Essay II xxvii 19-22] If the security videos convince a jury that this living human body is the one who pointed a gun at the teller, then my protestation that I have no conscious awareness of having the bank is unlikely to prevent my conviction in court, unless I can provide some over-riding evidence of my loss of consciousness.
Locke seems to have been little concerned with the difficulty of proposing an explanation of moral accountability that rests upon a criterion of personal identity that cannot be reliably applied by other observers. The eternal sanctions for obedience to divine law, he firmly believed, will be distributed ultimately by a deity to whom "the secrets of all Hearts" are perfectly accessible: although human courts may err, God will not. Besides, the most vital role of Locke's account is to link obligation and motivation in the deliberative process that takes place entirely in the first-person mode: it is my own present belief that I will be punished later that disinclines me to misbehave now.
[Essay II xxvii 25-26]