'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.

If the alternatives are living in the past and living in the future, we can only live in the present. If the alternatives are thinking about the past and thinking about the future and thinking about the present, we have choices. "Living in the present" is a cognitive psychological technique used, often successfully, by those who brood about the past or fret about the future. Concerns with the past or future may be appropriate (e.g. someone regrets a romantic choice or gets a worrisome medical diagnosis) or inappropriate (due to anxiety, excessive guilt etc); the technique works for all of them. Many people report that it helps them live a fuller and calmer life. For those who suffer from poor impulse control or psychopathy, however, it might be better to focus more on the future and the past and less on the present.

Individuation is the process of picking out individuals. We do this all the time in ordinary life. For example, if we were in a parking lot, we could individuate the cars in the lot. That is, from the group of objects in the lot, we could distinguish the individual cars. We wouldn’t have a difficult time figuring out, for example, whether there are five cars, or two cars or one car. We could distinguish each individual car. (Of course I haven’t said how we would do it, but it shouldn’t be hard to tell that story.) So there isn’t a problem of individuation for cars, at least in ordinary circumstances.

Your excellent question is how we individuate, that is distinguish, beliefs. Fred’s belief that the combined landmass of French Polynesia is roughly equivalent to the landmass of Rhode Island is a different belief from his belief that the total area (including the sea) of French Polynesia is roughly equivalent to the total area of Europe. Here we have individuated the beliefs by reference to the difference in the propositions which make up the content of the belief. But if we individuate beliefs by the propositions they are attitudes towards, it follows that beliefs about logically equivalent propositions are really the same belief. So Fred’s belief that if the boat sinks, everyone will drown, is the same belief as his belief that either the boat doesn’t sink, or everyone will drown. Contrast this with our car individuation task: Cars that look like separate cars really are separate. But here we have two beliefs that might, at least at first, appear to be separate beliefs, which, on this taxonomy, turn out to be different beliefs.

Some philosophers hold that mental states like belief are brain states. That is, they individuate beliefs such that if I’m in a particular brain state which, on their theory is the belief that p, then if we imagined a double of me on a twin-earth, who is physically identical to me, then, if he is in the same brain state, then he also has the belief that p. But some philosophers of mind, such as Hilary Putnam, and later Tyler Burge and others, have challenged this account, by arguing that two such individuals could have different beliefs, even while being in the same brain state. They conclude that we can’t individuate or taxonomize beliefs on the basis of brain states.

So to sum up: To individuate, we need a theory. We have a theory of what makes something a car, so we can distinguish cars in a parking lot. We’re less clear about what beliefs are, and different theories may individuate beliefs differently. If one theory of belief says there is one belief where another sees two, or two where another theory sees one, then those theories are in conflict, and the basis of taxonomy is still open.

The problem you are raising here is actually very nicely discussed in Derek Parfit's famous book Reasons and Persons (Part III). Parfit asks you to imagine tele-transportation, where your body is carefully scanned (and destroyed in the process), the data e-mailed to some destination, and a human being constructed at this destination who is an exact replica of you, including your memories and whim for hazelnut chocolate. You'd be scared to travel this way, but seeing that others do it safely all the time, you too do it and get used to it.

Now one can ask whether the person getting out of the machine at the destination really is the same person as the one who walked into another machine at the departure point. As Allen Stairs wrote back then, something can be said for either answer. But there's a third thing one might say: once the story's been told, there is not further question to be answered. You can say what you like about sameness, the important thing is that you really have no serious reason to avoid using this technology -- provided it works, of course!

One day you travel by e-mail, the technology actually works a little better than usual. The scan does not actually destroy the person at the departure point. So now we have two people, just as you imagined in your question. As you write: if they both were identical with the pre-departure person, then they'd be identical with each other -- and this they surely are not, seeing that they are miles apart from one another and having a heated conversation with each other on the telephone (about who gets to be with hubby and the kids).

When you used the technology in the old days, when the scanner destroyed the person at the departure point, you thought of e-mail travel as being just as good as taking the train. You may be a different person each time, strictly speaking, but why mind? Parfit examines the plausibility of this attitude for the branchline case, where you are the person who survived the scanning. He imagines that the scan did damage after all and that you are going to die rather soon. Can you be as cheerful about this, in light of the person at the destination point, as you used to be pre-scan in your previous e-mail travels? It would be hard to be cheerful like this in the face of imminent death, but Parfit makes a good case that you have reason to be. So have a look and see what you think.

If I have it right, your brother thinks he won't change much, because he thinks that people in general don't change much from teen years to adulthood. He then goes on to draw a conclusion: children (or at least, teenagers) aren't really any different from adults. So we have two questions. First, is the premise true? Is your brother really right when he says that people who have reached the ripe old age of 13 are pretty much as they will be as adults? That's not a philosopher's question as such, though I don't think I'm going out on a limb when I say that many people change a good deal after they get past their teen years. But there's another question: even if we granted your brother his premise, what about his conclusion?

It would depend, wouldn't it? It may be that people's basic personality (cheerful or prickly or inclined to fuss-budgetry...) is set by the time they reach their teen years. And it's pretty plausible that mannerisms are laid down early. But I'm guessing your brother thinks his argument gets him quite a bit more: the right to adult privileges. That's not so clear. Even if thirteen-year-olds are in many ways like the adults they will become, there's something else we can say: the parts of the brain that govern decision-making and planning (the frontal lobes) aren't ready for the corner office and the keys to the liquor cabinet. If you click here or here, you can see a couple of the many hits that a bit of googling will come up with on this topic.

This won't come as a surprise to anyone who's spent a lot of time around adolescents (not least the parents among us!) or has a good memory for their own teen years. The science is simply giving us the "why" behind something we already knew.

There are exceptions, of course. Some thirteen-year-olds are remarkably mature. Maybe your brother is one of them, though the fact that he thinks so doesn't exactly distinguish him from his peer group. But his rough-and-ready generalization doesn't get him where he wants to go, as he'll likely agree himself in 20 or 30 years.

Wow, you have come up with a case I love to use in my philosophy of mind to connect issues of personal identity to moral responsibility and "moral luck." I have students read the Oliver Sacks' case of Donald ("Murder" in The Man Who Mistook His Wife for a Hat). Donald took and, while high, killed his girlfriend. He had no memory of the incident (assume this is true for now) and was found not guilty by reason of (temporary) insanity. A few years later he was hit by a car and suddenly (re)remembered the gruesome murder (offering details only the killer could know). The case raises lots of questions: Is Donald on PCP (DPCP) the same person as Donald before (DBefore)? And is Donald at trial and the next few years (DTrial) the same as DPCP? And is Donald after recovering his memories from the accident (DAfter) the same as ... and DTrial and DBefore??

And beyond these questions about personal identity, there's the question of moral luck: assuming that Donald before (DBefore) had no more reason to think he'd become a killer than anyone else planning to take PCP, should DTrial (or DAfter) be charged with murder (DPCP seems to have intentionally killed his girlfriend, as in second-degree murder)? Or is Donald on PCP such a different person that it is only fair to blame him for doing something as stupid and illegal as taking PCP but not for murder?

So far, I've basically just re-iterated your very interesting questions, but I thought the parallels were interesting. Now, how to answer them? Well, everything depends on your theory of personal identity. If you hold John Locke's memory (or same consciousness) theory, as you seem to, then it seems that DTrial should not be punished for what DPCP did since he can't remember it, but as you suggest, DAfter could be punished for what DPCP did--unless you want to bring in the moral luck worry and say that even though DAfter remembers it, he shouldn't be blamed for more than taking PCP since DBefore had no reason to think he'd do what DPCP did! Except you might think there is something bad about DBefore's character that predisposed him to murder when he loses his inhibitions! Or you might want to charge him with manslaughter as we do with drunk drivers who kill (another case of moral luck since the drunk driver who doesn’t kill may have just gotten lucky someone didn’t cross his path). And then there's the (epistemic) problem of how we can know whether DTrial is faking it or not (the problem of other minds rears it's ugly head). Even Locke suggested that we must punish the man who commits a crime while drunk and says he doesn't remember it because we can't be sure, and we have to deter others from trying to get off by committing crimes while drunk (well, he said something like that). So, as a general rule we may need to punish bodies for what they do even if they claim not to remember doing it. Or we may decide to punish bodies because we hold a bodily criteria for personal identity. If I read Derek Parfit right, he seems to suggest that our practical interests (such as legal responsibility) will set the boundaries of the conditions for personal identity (e.g., we'll just have to stipulate what to say about weird cases) rather than there being a metaphysical truth about personal identity which we then apply to our practical interests (such as legal responsibility).

I better stop before I try to deal with your case of the murderer copying himself.

Some of your difficulty -- very reminiscient of Leibniz, by the way -- may be caused by the word "different." Take a very simple case, two water molecules perhaps. Are they different? In one sense, they are exactly the same. Yet in another sense they are different or (perhaps better) distinct. You can tell that they are not the same in this second sense by counting: there are two, not one. And you can tell this, in turn, by attending to their space-time locations.

Similarly with your more complicated example. At any given time, there are two distinct locations at which a human being with this DNA is located: you at one place and your clone at the other. If he is living on earth, he's likely to be a bit different from you due to what the two of you have eaten and experienced. But he may be living on a planet that is an exact replica of this one, and his life may then mirror yours exactly with him thinking and doing exactly what you think and do, perhaps even simultaneously. He would still be distinct from you by virtue of his location. You are here and he is there.

Here's another thought experiment that philosophers sometimes appeal to in this context. Suppose someone invents a teleportation machine (like in Star Trek). The machine scans your body, vaporizes it, and then recreates a molecule for molecule duplicate somewhere on Mars. Would you survive this process? That is, would the person on Mars be the same person who stepped into the machine on earth? Or would you cease to exist, only to be replaced on Mars by someone who is exactly like you? If personal identity is just a matter of physical continuity, then you probably don't want mess around with teleportation.

It isn't entirely obvious that I, say, could have been female from conception, and the assumption that the fertilizing sperm was different certainly leaves that possibility open. But if I could have been female, then your great-great-great grandfather (let's say) could have been female, and one of your great-great grandwhatever's parents would have had a hard time conceiving a child together.

Maybe that isn't the sort of possibility you had in mind. But it's not obvious how to restrict it and still get plausible results. Let's suppose you could have been the result of fertilization by a different sperm. What's so special about the sperm? Why not a different ovum, too? But now consider that other ovum and sperm. The latter could have fertilized the former even if the ovum and sperm from which you were actually formed still got together. But then are you your own twin? I don't think so. So it doesn't look as if you could have been the product of a different ovum and a different sperm. But if not, then why should only one of them be required? It's not obvious how to answer this question.

Our atomic structures don't remain the same over time. Some changes in structure sustain personal identity. Others don't, such as death. And some would argue that certain types of changes in neural structure would result in person a being replaced by a different person, b, as owner of the same body (say if b's neural structure were copied in a's brain).