From Bryan Magee, The Great Philosophers: An Introduction to Western Philosophy, Dialogue 6: Michael Ayers on John Locke (pp.134-6)
Magee: The body-mind distinction raises another important question. If Locke thinks that all material bodies, including therefore our own, are in their inner nature mysterious to us, and that minds are equally mysterious, what is his view of personal identity?
Ayers: The discussion of personal identity is one of the most original and interesting parts of the Essay Concerning Human Understanding (1690).
He agreed with Descartes that I know that I am a thinking thing, but he held that I don't know my nature, because I don't know what nature a thing has to have in order to be able to think. Followers of Descartes held it a very powerful argument for their view that it explained personal identity. For them the identity of a person even in life could not be determined by the body, since matter is in continual flux. So it must be determined by the identity of the soul. The same soul can exist after death - indeed they argued that it followed from the soul's being immaterial and unextended that it is also by nature indestructible. So at the resurrection personal identity would go along with the same soul. Now Locke started from a different consideration, which is that inimortality has to be personal immortality. The whole point of immortality is, to put it bluntly, reward and punishment. But unless the thing that is being punished in the after-life is conscious of the deeds that it has done in life on earth, then Locke thought that punishment has lost its whole point.
Magee: It would be the equivaient of a different person's being punished.
Ayers: Right. Suppose that we grant that there is such a thing as an immortal, immaterial soul; suppose we grant that that is what receives punishment. If that soul has no recollection of what happened on earth, immortality loses its point. So what really matters, in Locke's view, is not the supposed immaterial soul, but consciousness, the unity of consciousness, whatever is its natural basis.
Magee: And the continuity of consciousness.
Ayers: The continuity of consciousness, that is, the individual's consciousness of its past. And of course in this life what matters is the thought that it's going to be oneself who is going to get punished in the world to come.
Magee: For Locke, then, memory is the key to personal identity: it is more than anything else the fact that I carry within me a living awareness of my own history that makes me the person I am.
Ayers: Yes. Locke doesn't deny or doubt that the memory will have some sort of substantial basis. His point is we don't know what that is. Really the point of his whole argument is to allow for the possibility of immortality without going against his anti-dogmatism, without accepting the immaterial soul of the Cartesians as something of which we have knowledge. But what makes his theory so interesting and important, even today, is that it introduced into modern European thought the idea of the self as constituted by a connected, if interrupted, stream of consciousness. That scandalised the orthodox at the time, but has remained ever since a powerful ingredient of the way we think about ourselves.
From Robert Langbaum, The Mysteries of Identity: A Theme in Modern Literature (NY: Oxford University Press, 1977), pp. 25-27
Identity is "the sameness of a person or thing at all times or in all circumstances." As a term in philosophy identity used to apply mainly to the unity of objects, especially through an expanse of time: "a single object, plac'd before us, and survey'd for any time without our discovering in it any interruption or variation, is able to give us a notion of identity." The word did not take on its current psychological denotation, it did not begin to be applied to the self, until the unity of the self became problematic. As long as men believed in a soul created and sustained (continuously known and seen) by God, there could be no question about the unity of the self. It is significant that identity is first used to mean personal identity by the empiricist philosophers Locke and Hume, who use the word identity to cast doubt on the unity of the self.
The term in this sense is not used by Descartes, who might be considered the founder of modern philosophy and the last philosopher to take the unity of the self as axiomatic. But Descartes' "I think, hence I am" so amputates the self by reducing it to consciousness that, despite his intention to substantiate the self, Descartes has probably done more than Locke and Hume to kill it off, as Beckett's use of Descartes suggests. Most of the nineteenth- and twentieth-century writers I shall discuss recur to one or another of these Enlightenment philosophers or their successors (Bradley on his empiricist side serves Eliot the way Locke and Hartley serve Wordsworth), as defining the self in a way they both accept and resist.
Hume, in his section "Of Personal Identity," [in Treatise of Human Nature] raises most of the issues about identity that I shall discuss in this book. "There are some philosophers," Hume begins,
who imagine we are every moment intimately conscious of what we call our Self; that we feel its existence and its continuance in existence; and are certain, beyond the evidence of a demonstration, both of its perfect identity and simplicity.
But the self, Hume argues, is not experienced. What we experience are successive, changing impressions all of which are supposed to refer to the self:
Pain and pleasure, grief and joy, passions and sensations succeed each other, and never all exist at the same time. It cannot, therefore, be from any of these impressions, or from any other, that the idea of self is deriv'd; and consequently there is no such idea.
The issue here, which we will see repeated over and over, is whether we experience successive selves rather than any one self.
"I never can catch myself at any time without a perception, and never can observe any thing but the perception." The self then is equivalent to the conitents of its perceptions and ceases to exist when it ceases to perceive, as in sleep or death. Hume has no concept of unconsciousness, and therefore does not allow for a sense of self in sleep or dreams.
We "are nothing," he says, "but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement."
We arrive at the sense of self through error, through the process of association; we pass insensibly from the idea of succession to the idea of identity, because the imagination feels the same in conceiving these opposite ideas (here Hume anticipates nineteenth-century dialectical thinking). In order to justify this absurdity, "we feign the continu'd existence of the perception of our senses, to remove the interruption," or we imagine "something unknown and mysterious, connecting the parts" (here Hume anticipates romantic organicism), and thus "run into the notion of a soul, and self ... to diguise the variation." The self, in other words, is a necessary fiction. Hume anticipates and rejects the dialectical logic and the organicism by which later generations will try to solve the problem of the self as he defines it.
Hume concludes that identity is not in the different perceptions themselves, uniting them, "but is merely a quality, which we attribute to them, because of the union of their ideas in the imagination, when we reflect upon them." The self is a retrospective construction of the imagination, and for this reason "memory not only discovers the identity, but also contributes to its production." Only through memory can we create the self by seeing continuity between past and present perceptions; only through memory can we conceive "that chain of causes and effects, which constitute our self or person." Hume does not deny the self as an operative presence; like Locke he insists that it is a fabrication achieved through association, imagination, memory - especially memory. ... Memory above all [is] the creator, the artist-fabricator, of self.