If I buy myself an old car, it will need working on. Bits of engine, panels and upholstery will need to be replaced, and more will need replacing over time. Eventually, if the car is kept long enough, there will be no part of the original car left. In one sense, it will still be the same car, but in another it will be new, since all the parts will have been replaced at some point.
Is this true of people too? Whilst I’m going about my day-to-day business, my body is busy doing its stuff - old skin is being replaced with new skin, blood is constantly being renewed, dead cells are being replaced with new cells, and so on and on.
Presumably, if we get right down to it, there are new molecules being taken into my body to make up new cells, and old ones are leaving with waste products. So, over time, the original me is being replaced with new bits.
Will there be a point where virtually none of me will be composed of any part of the original me (say, when I was born)? That is to say, can we predict how long it will be before very few original bits of me are left unreplaced?
The difference between you and the car is that you’re making your own new parts. You started out as a single-celled organism, and your cells have been dividing and subdividing ever since. Is any of your current cells them the original cell? Are they all the orginal cell? Depending on how you look at it, you ceased to be the original you when your single cell divided into two, or you will never cease to be the original you by this process.
I thought this was going to be a thread about changing personality; I hope it isn’t too much of a hijack to mention that, looking back (and particularly on reading things that I wrote a long time ago), I sometimes have difficulty in distinguishing the ‘me’ of the past from a complete stranger.
It also depends on what you mean by original. I think I’m right in saying (and no cites yet, but I’m working on it) that all the cells in your body are replaced every 7 years, with the exception of your tooth enamel, which is with you for life. So, assuming you’re old enough to have had your adult teeth for more than seven years (!), then you could say that you’re not the same person you were then!
The only nagging doubt I have is brain cells, not sure if they’re replaced also. Like I said, no cites but I’m working on it. (Damn, that’s more cells in my fingertips that I’ll never see again…)
From what I have read on the subject (though this was awhile ago, so prevailing opinion may have changed) the neurons in your brain do NOT die off and get replaced in the same way as other cells. Neurons can certainly die, but they are on average much longer lived than most cells in the body, and the interconnections between them are fairly permanent. So if you judge by neuroanatomy, “you” are a little more durable than if you judge by gross anatomy.
I’m no particle physicist, but it seems to me that the sub-atomic particles that make up your body would be completely replaced in very short order, no?
Wow - 7 years. I wasn’t expecting anything of that short time-scale.
What ForgottenLore says about neurons is reassuring - without this bit of continuity, it would indeed be fair to say that the me that exists now will no longer exist in any physical sense in 7 years time (well, except for my tooth eneamel :)).
So, I am replacing the car bit by bit, but luckily the driver remains the same.
Weird. I just got done watching the movie Waking Life (great movie btw) in which this was discussed and I was about to ask the very same question. I’m with Mangetout though…the 7 years thing does sound pretty suspicious.
Not a meaningful question. Given two subatomic particles it’s not even in principle possible to determine which one is which. Certainly they may have different energy, momenta, etc. but were you to take two protons, swap their positions and momentum, it’s not just that there is no observable difference: There is no difference.
IANABiologist, but here’s my take on the philosophical side of things:
It’s not really meaningful to think of yourself as a collection of cells. It’s true, but that doesn’t mean it’s very useful. You are defined by the structure those cells form. If you took all the cells in your body and just dumped them in a big pile, that pile would in no way be you even though it has all your cells, no more than if you were reduced to your base elements and these were all piled up. Similarily were I to instantly replace all the cells in your body with ones that weren’t exactly the same but behaved in an identical fashion, you would still be the same person. You probably wouldn’t even notice anything.
I know, this doesn’t precisely answer your question, but it’s interesting and semi-relevant nonetheless.
If every part of me gets replaced in 7 years (or however long it is, if this is indeed a myth), then I guess the natural next question is “well, what am I”.
I suppose I am the mind (brain?) inside the body; the body is just a vehicle, and so replacing it piecemeal doesn’t replace me.
I may be mistaken, but I thought the last issue of Scientific American mentioned that neurons may in fact be replaced as other tissue is.
FWIW, IIRC Pausanias mentioned that there were a couple of what we’d call “historical ships” preserved in ancient Greece which over the years had been completely replaced piece by piece several times over; this was occasionally cited as a metaphor for the puzzle posed here. Are they the ships or not ?
If you are equal to or (hopefully) greater than the sum of your parts, then the assemblage of “new” parts functioning as you is you. If not, then the toilet, the bath drain, and the dust mites got the better part of you a long time ago.
That 7 year bit… I believe that is how long it takes to “replace” the majority of the cells in the slowest growing or changing parts/systems in the body - I do remember watching a human biology bit on TV talking about how bone cells are replaced, and I think that was the time they gave… you essentially grow a new skeleton every 7 years. Other cells grow much faster, like your hair and male… uh… reproductive cells. Of course it’s an on-going process and doesn’t happen the same way as all at one replacing the drywall in your house.
Only if the two particles are swapped directly and instantaneously… but that’s not how it happens in biological systems.
Rather, you have molecules breaking down, and being subsequently replaced. There is not even necessarily a one-to-one correspondence between the original particles and their replacements. So, it does make sense to talk aobut the original particles being supplanted by new ones
Even if whole neurons don’t die off and get replaced, individual cells are always exchanging molecules with the blood stream. This “eating” and “excreting” means that at least some parts of the cell are getting replaced continuously.
IANAB, but I would guess that the average replacement time varies widely for different parts of the body. Skin and hair are replaced quickly, while teeth and bones take much longer (if at all) to change.
