DNA cannot very well serve as a sufficient condition for personal identity over time, otherwise identical twins would each be identical with both their past and future selves. Can DNA serve as a necessary condition for personal identity over time? Imagine a futuristic machine that introduces a minute and meaningless change to your DNA (difficult, I realize!) at 4pm today -- a change that would not result in any noticeable changes in your feelings, memories, conduct, appearance, etc. Would it be credible to say that the person after 4pm is a different person from you? These are, I think, among the reasons philosophers would give for not taking DNA to be a good answer. But then good answers are not easy to come by for this question.

A CLASSIC case! This is a major issue going back to ancient philosophy. The example used then was the ship of Theseus (a Greek hero). Imagine you have the ship of Theseus and a similar ship side by side. First you switch one part (the mast, say). Is the ship of Thesus still the same? Many of us want to say 'yes,' but then we get puzzled as more and more parts are switched until eventually it seems the ships have changed places. One route that philosophers have taken might bring peace to your family: some philosophers distinguish a strict sense of identity from an identity that is "popular and loose." On a strict view, you are right. Any object with parts is not the same if even a single part is removed. This is technically called mereological essentialism. According to mereological essentialism, your father's body today is not identical with the body he had as a boy. You might even suggest to him that while he went to first grade, that (pointing at his body) did not. You can retain mereological essentialism while also allowing that sometimes we can and should meaningfully speak of sameness of identity that is not strict. In the later, though, there will be different conventions that come into play. So perhaps there is a way to allow that you both may be right? If push comes to shove, however, I am on your side on this. For a defense of our view, see the book Person and Object by Roderick Chisholm.

"Is the same as" is ambiguous. It could mean "same thing" or "same kind of thing" or even "same thing but not necessarily same stuff". Your father is speaking in the last sense--the glasses look the same, and moreover, the 3 pairs of glasses are built through successive fixes. You can avoid the verbal paradox by asking "the same in what respect?" The glasses are not the same stuff, and they do not have all the same physical properties (such as weaknesses in the glass and frame) but they are the same style and they are constructed successively out of one another.

Great question and suggestion! While some philosophers (most notably John Locke) have claimed that the key to personal identity is memory, probably the majority of philosophers today do not. Most grant that you might endure as the self-same subject despite all kinds of memory loss and replacement.... So, if it is a fact that no one does remember their past lives, it would not follow (on many accounts of what it is to be a self), this may be only a problem of epistemology and we cannot from that alone assume that reincarnation is false. Probably one reason why some today think reincarnation cannot occur is because they think that for reincarnation to occur, a person (self, subject, soul, mind) would need to switch bodies. Those of us who are dualists or who think there is something to persons more than the material body, may well grant that it is possible for a person to come to have a new body. But materialists who think that you and I are our bodies will have grave doubts about whether we can survive the destruction of our bodies.

You are a good guide here since you are undergoing the changes. Presumably you have initiated this process because you feel that you are really not the gender you started off as, and so your notion of personal identity was quite complex even before the process got underway. Clearly we change all the time, and sometimes so radically we come to believe that we are quite different from how we were in the past. You are in the interesting position of perhaps feeling that you are finally approaching becoming the sort of person you "really" were all the time, and you are thus in the best position to report on how your feelings make up this changing self-perception. Self-identity is clearly far from a simple notion and nothing evidences that so much as your course of action.

As I posted this, I saw that Donald had offered a similar reaction. But since I'd already written this up...

It's a very interesting topic you've raised, and one on which philosophers have written a great deal. My view fall into the blunt, even philistine category, but I'll point to other views as well.

Let's begin with your final question: is this worth getting worked up about? My answer is that it's not. What's at stake is whether some highly abstract, theoretical, and hard-to-settle metaphysical claims are true. Even if they are, life will go on as usual. You'll still experience things, remember things, look forward to things, make plans, carry them out, and in general live a human life. If there's some sense in which there isn't a single "person" that lives this life, the most psychologically healthy response is probably a shrug.

You ask whether our consciousness continues while we sleep, or whether, on the other hand, it stops and restarts. One way to read that (probably not the best way) is whether there's some mental "thing" that exists continuously throughout our lives. If that's the question, the best answer is that there's no good reason to think so. But unless we work ourselves into a philosophical lather, there's also no good reason to think that there has to be such a thing for you to be the same person who went to sleep last night. On another, probably better reading, the question is whether there's always some sort of conscious experience going on inside us (I'm treating dreams, for example, as conscious experience for this purpose) even when we're deeply asleep. Though I don't know for sure what the answer is, my impression is that there isn't. But it's not clear why I should care. Why think that for me to persist, there must be an unbroken stream of experience?

People are complicated. We have bodies with brains. The brains, it seems, are what lets us have thoughts, feelings and experiences. The goings-on in our bodies (brains included) fit together in ways that are deeply fascinating. We've learned a lot about how the more straightforwardly biological aspect work, and also how the minds that our brains give rise to work. Some of the things we learn lead us rightly to think of ourselves differently. For example: we know that we're neither as rational nor as psychologically unified as we might have thought we are. That's interesting and important. It also may be better for some purposes to think of ourselves as a complicated set of processes than as "things," but even if it is and even if we do, life goes on.

The Buddha taught a doctrine of "no self," which seemed to mean that there is no persisting inner object worth being called The Self. That's quite plausibly true. And Buddhism's view that we get out of whack when we get attached to the passing show of experience, wanting to control it and bend it to our will, is in my experience a wise teaching. Matters of living wisely aside, many philosophers have offered views of personal identity that have a lot in common with the Buddha's; the 18th century Scottish philosopher David Hume ad contemporary philosopher Derek Parfit are two notable examples. But very few of our human concerns call for getting the right take on tricky matters of metaphysics. When you think about it, that's a very good thing.

There are arcane theoretical reasons for thinking that you are not the same person when you wake up. There are also arcane conceptual scenarios that prevent you from being absolutely certain that you are the same person. These considerations are no match for your everyday certainty that it is you, even if they are able to cause short-term anxiety. Your everyday certainty will return, with a faint residue of wonder that we cannot absolutely prove the things we know. That residue is part of the point of doing philosophy. It helps save us from the dogmatic arrogance people are prone to. Your professor was partly joking and partly trying to push you toward this philosophical lesson.

I love that video! Thank you for your excellent question.

Of course, you are correct in saying that there is no sharp demarcation between the hand and its air around it. A water molecule that is part of the hand may at some point evaporate into the surrounding air. There is no particular moment at which the molecule leaves the hand and becomes part of the atmosphere. Its chemical bonds to other "hand" molecules weaken gradually, its distances from those molecules increase gradually, and even if these quantities do not change continuously (in the mathematical sense), there is no magic bond strength or distance at which the molecule officially leaves the hand and joins the air.

That being said, the fact that there is no sharp distinction does not guarantee that there is no distinction at all -- that "separateness is an illusion inherent to the experience of beings at a macroscopic scale". After all, there is no sharp distinction between night and day -- yet night is not the same as day. It may not be illusory to think that some molecules do not belong to the hand, some do belong to the hand, and some occupy an intermediate zone (a "gray" area). Separate entities can exist even if there is such an intermediate zone.

Moreover, in the course of solving a given physical problem, we might have to make more explicit what the difference is between the hand and its surrounding air. We might have to stipulate that a molecule belongs to the hand if and only if it feels a force greater than x towards the interior of the hand. Such a sharpened-up distinction may be somewhat arbitrary in the specific x chosen. But it might be serviceable for the purposes of a given physical problem. There might be a range of equally good ways of demarcating the hand from the air for the purposes of that problem. I'm not sure that such a case is best characterized in terms of separateness being "an illusion".

A thought experiment akin to the one that you propose has been deployed by Derek Parfit, in his classic book, Reasons and Persons, which I highly recommend if you're interested in this topic, in order to argue that personal identity is "not what matters." According to Parfit--I'm simplifying somewhat--if some agent has a perfect duplicate who is psychologically continuous with the agent, then, according to Parfit, even if the agent dies, and therefore the agent's consciousness does not continue, and so s/he does not continue to exist (Parfit is therefore answering question to the negative, and admitting that you won't survive) what's important, namely one's plans, projects, etc., will be continued by the agent's duplicate, and that, again according to Parfit, is more important than the survival of one's consciousness--indeed, in such a case, one shouldn't be worried about dying, because what one values will be carried on (albeit only by a psychologically continuous duplicate of oneself). Now Parfit finds these considerations reassuring: are they? Is what matters to being the person that one is simply that one's plans, projects, etc., be continued? Consider the following extension of the thought experiment. Suppose that one is married, and that, unbeknownst to one's wife, one's perfect duplicate will take one's place upon one's death. Ex hypothesi, she won't be able to distinguish the perfect duplicate from the person with whom she shared her life up until that person's death and replacement by a perfect duplicate. Now suppose that she were to discover, somehow, that she is now living not with the person with whom she had shared her life up until the replacement, but with a replacement--albeit one physically and psychologically indistinguishable from the original: should she be unconcerned? Reflection on this question, may, I think, lead to a refining of one's intuitions regarding the importance of identity.

I'm with you. I don't see any reason to suppose that some future person who happened to share your DNA would be you, no matter how similar the course of their life might be to yours. And that's even before we get to the question whether time in infinite (quite possibly not) and whether, if it is, it has the implications that this argument supposes.