What used to be called multiple personality disorder (MPD) is now called dissociative identity disorder (DID) in DSM, the Diagnostic and Statistical Manual of Mental Disorders, and the change in terminology may reflect a possible change in thinking. A personality (e.g. a television personality) is in one sense a person, but generally I and my personality are not the same (Same what? Same entity, presumably, if that is any help!) In any case, since my personality might change considerably over time, while I remain the same person or the same individual (I assume that individual and person, as they are applied to human beings, are equivalent concepts), the two cannot be identical. And most of us do exhibit slightly different personalities, for example at home and at work, or with friends and with superiors. Those who have a mixed ethnic or national heritage can sometimes find that they have two personalities, say one English and one Indian. They may feel the pressure on the integrity of the person. But since personality and person really are distinct concepts, they should not feel this pressure. After all, if Mr. X, who is half in Indian culture and half in English culture - as though there were only one of each! - commits some awful crime, then it is Mr. X who will be punished, no matter which personality he is manifesting or which country he is charged in. The two concepts, of personality and of person, could without too much of a stretch be called the psychological and logical (or metaphysical) concepts, respectively. Now the question is whether a change in the application of the psychological concept forces a change in the application of the logical or metaphysical concept. The answer to this seems to be negative, as I have said. Certainly if we say that in DID we are confronted with two or more individuals or persons, individuals falling under the category of person, we are implicated in a psychological concept of personal identity rather than a physical one. The criterion of identity is not the body, but the consciousness. This was Locke's view, and in the Essay concerning Human Understanding he proposes a celebrated thought-experiment in which the consciousness of a prince, and with it the soul of the prince, is transported into the body of a cobbler. The question he asks is whether the person before us is the cobbler or the prince, and his answer, relying on the consciousness criterion, is that it is the prince, although he also says that the man or human being is the cobbler. Now all we have to do is to imagine the prince vacating the cobbler's body, and the cobbler's consciousness taking up residence there again, and we have a description of DID as it might be seen by the proponent of the consciousness criterion. Overall, given that the concept of a person is partially a forensic or legal one ('It is a Forensick Term . . .'), as Locke observes, as well as a metaphysical and logical one, and given the improbability of Locke's scenario, it seems better to say that in DID what we observe is an oscillation between personalities, not individuals, though one marked by the extremeness of the difference between the personalities and the lack of communication between "them", manifesting as memory loss. This is supported, I think, by a consideration of the proposed origins of DID in childhood trauma.
DNA does change. There are "point mutations", for example, in which say a single nucleotide changes, say from guanine to cytosine. . . . CTG TCA . . . becomes . . . CTG GCA . . . If there is a strand of DNA that suffers such a change, is it then not the same strand of DNA? This is exactly like the question whether persons become different persons if they lose say half a finger. And now we have the problem of DNA identity. When are two descriptions sufficiently similar to count as descriptions of the same strand of DNA? Anthony Quinton has the general issue right, in a 1962 article in the Journal of Philosophy called "The Soul": 'No general account of the identity of a kind of individual thing can be given which finds that identity in the presence of another individual thing within it. For the question immediately arises, how is the identity through time of the identifier to be established? It, like the thing it is supposed to identify, can present itself at any one time only as it is at that time. However alike its temporally separate phases may be, they still require to be identified as parts of the same, continuing thing.' By the way, 6% of identical twins do not have identical DNA, so the members some pairs of identical twins would be metaphysically identical and some would not.
'We can only live in this "here and now" moment . . . in fact , there is no way we can ever live out of it . . . is it not?'
I am not sure what is supposed to meant by living in the present instant ("moment" I think has more to do with action). Living at an instant seems as impossible as living at some other time, because there isn't even time to draw breath in an instant. In any case I do not believe that there is something called "the present instant", so I don't see how we could live in it (at it?)
It (the present instant) is an abstraction, and it is not, in reality! I do believe there are present times, though, such as the present day or hour. The trouble with the instant is that it is not a time.