Of the three basic kinds of complex idea, relations are the easiest to understand. The mind can consider any idea as it stands in relation to any other. By observing the similarities and differences, the mind derives further ideas, ideas of relation. For instance, we might compare our simple ideas of two patches of color and notice that one is of a different size than the other, thereby getting the idea of bigger and the idea of smaller. Or else, we might compare our ideas of two people and get the ideas of father and son. Our ideas of cause and effect, which Locke examines at length in chapter xxvi, are produced by noticing that qualities and substances begin to exist and that they receive their existence from the operation of some other being. We call a "cause" whatever produces any simple of complex idea to come into existence, and an "effect" whatever is produced. Our ideas of moral relations, which Locke turns to in chapter xxviii, are produced by comparing our voluntary actions to some law. It is Locke's third and final category of relational ideas, ideas of identity and diversity, that is of great importance to the history of philosophy. This is the topic of Chapter xxvii. It is in the context of this discussion that Locke presents his theory of personal identity, that is, his theory of what makes us the same person over time. According to Locke, remaining the same person has nothing to do with remaining the same substance, either physical or mental. Instead, personal identity has only to do with consciousness: it is by the consciousness of one's present thoughts and actions that the self is conceived, and it is through the continuous link of memory that the self is extended back to past consciousness. Locke's argument for this claim rests on his idea of identity, which is defined in terms of a comparison between something presently existing and the existence of that thing at an earlier time. This notion of identity stems from the basic principle that no two things of the same kind can exist in the same place at the same time, as well as the extension of this principle that, therefore, no two things can have the same beginning and neither can any thing have two beginnings. Things retain their identity, then, as long as they do not become essentially altered because once something is essentially altered, it has a new beginning as a new thing. In other words, identity is retained through continuous history. Of course, to remain essentially unaltered has a different meaning for different ideas. Locke separates the idea of a substance, the idea of an organism, and the idea of a person. The identity of these three types of idea is determined by different criteria. The identity of a material substance consists merely in its matter; a mass of atoms retains its identity as long as the number of atoms remains the same. The identity of living organisms cannot be tied to matter because both plants and animals are continuously losing and gaining matter and yet retain their identity. The idea of a living organism is of a living system, not of a mass of matter, and therefore it is only the living system that must remain intact for the identity to remain the same. Locke chooses the word "man" to refer to that aspect of the human being that denotes him as a type of animal. With this definition of man, Locke is able to claim that the identity of man, because it is just a particular instance of animal, is tied to body and shape. That other aspect of the human being, the human as a thinking, rational thing, Locke calls "person." The identity of person rests entirely in consciousness. A person is defined as a thinking thing, and thought, as we have seen, is inseparable from consciousness (remember Transparency of the Mental). It is, therefore, in consciousness alone that identity must exist.
Though it might be hard to believe, since personal identity has become such a standard problem in the philosophical repertoire, Locke's discussion of the subject was the first of its kind. Though other philosophers treated the issue of identity through time (the Ship of Theseus is a prominent example), Locke was the first one to separate out the specific issue of personal identity from the larger topic of identity in general. Locke's treatment of personal identity might seem counterintuitive to a lot of people, especially his claim that consciousness, and therefore personal identity, are independent of all substances. Notice, however, that the claim is not that consciousness can exist independent of a body or a mind, only that there is no reason to assume that consciousness is tied to any particular body or mind. Still, there is no reason to assume, on this view, that consciousness cannot be transferred from one body or mind to another (think of a science fiction example where all of one's thoughts are transferred to a computer chip, so that consciousness moves from the mind to the computer). That consciousness exists independent of material substance (i.e. body) is the more intuitive notion. Locke gives an example to illustrate just how intuitive this notion is: When a finger is cut off from a man's hand, it is clearly no longer a part of his consciousness; he is no more conscious of any effects on this finger than he is conscious of effects on any other man's finger. This is true not only for parts of the body but for the whole body as well, Locke insists. If the consciousness of one man were somehow transferred into another body so that the second body now contained all the memories of thoughts and actions that the first man once contained (but does no more), the person would now inhabit the second body and not the first. What is much less intuitive is Locke's claim that a person's identity is separate from any immaterial substance (i.e. mind) as well. After all, consciousness is inextricably connected to thought, and the mind is defined as the thinking thing. Consciousness, though, Locke insists, is not tied to any one mind even if it does require some mind or other. In addition to being somewhat counterintuitive, the claim that consciousness is independent of any mind raises some thorny problems. As a really existing thing, consciousness must either be a substance or a quality of a substance. Since Locke admits that consciousness cannot exist on its own, but must be part of some mind or other, it seems likely that consciousness is a property that belongs to minds. It is not clear, though, that a property can simply be transferred from one substance to another. A property belongs to a substance in a very intimate way. To say, then, that consciousness does not belong to any one mind in particular seems to indicate that it is not a property. If it were a substance, though, it would be able to exist independent of any mind at all. Locke does allow that he is not sure whether consciousness can, in fact, be transferred between thinking things, but he dismisses the practical question as irrelevant. However, this practical question might hold part of the answer to the nature of consciousness: whether it is just a property or something more substantial. Locke's theory of personal identity is also plagued by other problems. For instance, imagine that a man commits a crime, but at the time of trial he does not remember committing the crime. Would Locke be forced to say that the man who committed the crime was an entirely different person from the man on trial? Locke would probably respond that so long as the man on trial had some memories connecting his consciousness to the consciousness of his self at that earlier date, he could still be considered the same person, regardless of whether he remembered the specific crime. Consider, though, another example: An old man who cannot remember anything about his youth. Is he a different person from the one who lived his young life? Thomas Reid formulates this type of consideration into an objection that reveals that Locke's theory of identity is actually inconsistent. Imagine a man in three stages of his life, the objection goes, childhood, middle age, and old age. The middle-aged man can remember his childhood, while the old man can only remember middle age. According to Locke's view, the middle-aged man is the same person as the child, and the old man is the same person as the middle-aged man, and yet the old man is not the same person as the child. This, of course, is logically incoherent and shows that Locke's view is untenable as it stands. Reid goes on to modify Locke's view in a way that remains the basis for many further theories of personal identity: All that is needed to retain personal identity is some link of continuity. Though the old man may not remember his youth, his connection to the middle aged self who does have these memories is sufficient to link him to all parts of his life. Though Locke's theory of personal identity fails, it is significant both for being the first attempt at such a theory, as well as the theory upon which all further attempts have built.