Some will say that 'you' are your body. That everything that happens to 'you,' happens to your body, and vice versa. That you ought to identify, therefore, with your organs and tissues, your face, your DNA, and your brain — in other words, the totality of your biological organism.
Certainly there's a sense in which you do, already, identify with your body — behaviorally, that is. You'll go to great lengths to avoid losing an arm or a leg. And if you do lose one, it'll feel like you've lost a part of yourself. It won't be the end of the world; clearly, 'you' will still exist. But you'll feel less wholly yourself than before you lost the limb. The case is even stronger, of course, if you were to lose part of your brain.
I'm not suggesting that you take this road, mind you. To identify with your biological organism is appealing, certainly, but also problematic. In particular, the body is awfully hard to pin down. If you are your body, where do 'you' end? Where does your body leave off and 'the rest of the world' begin?
There's just no good answer to this question.
Boundaries of the body
Does your body end at your skin? Are you (in Alan Watts's memorable phrase) a "skin-encapsulated ego"?
If so, what do you make of your pores? Do you 'round up' and call each concavity a part of 'you' (along with the air inside it)? Or is each pore a little dent in your self-boundary?
(This is just the beginning of a very hard line of questions. Might want to brace yourself.)
What happens when you breathe? Each breath brings oxygen in and ushers carbon dioxide out. Do 'you' grow when your lungs inhale and shrink when they exhale?
What about your hair or fingernails? Are they part of 'you,' part of your body?
Why is it that you identify with your epidermis, but not with the food you're about to eat? If anything, food is on its way in, soon to become part of your body. Skin is on its way out.
What if you had a tapeworm living in your intestine? Would you consider it part of 'you,' or some kind of foreign invader? What about the bacteria living in your gut, your intestinal flora? Evidence suggests these are important for your health. When they're destroyed by antibiotics, for example, the effects on your health can be disastrous.
Does it matter whether you think of these creatures as parasites or symbionts? What if the scientific consensus changes, and what was once considered a parasite turns out to be a symbiont, or vice versa? Can your identity really change on such a whim?
What about your mitochondria? Are they 'you' or are they 'other'? Why do you seem to identify more with your nuclear DNA than with your mitochondrial DNA? Sure, mitochondria may have originally been a separate prokaryotic species — but that was well before humans came into the picture. How intertwined do the fates of two symbionts have to be before you'll consider them a single organism?
(And — by the way — what does it say about these questions that no one before 1900 would have even been able to ask, let alone answer them?)
What about cancer? Is that you? When a cell throws off its yoke and starts to rebel against the body politic, can you just revoke its citizenship?
Boundaries of the brain
But OK, maybe you think it's only your brain that matters. The rest of your body is incidental — just a fleshy robot controlled by the 'self' enthroned in your skull.
Fair enough. But note that you're just kicking the can down the road, leaving yourself to puzzle over a different set of boundaries: Where does your brain end and the rest of the world begin?
Imagine a photon from a remote galaxy heading toward Earth. When does 'it' become part of 'you', i.e., your brain? As soon as it enters your eyeball? When it tickles a rod on your retina? When the electrical signal arrives at your visual cortex? When you perceive it as a flash of light? Or could it have already been 'you' the moment it left its distant birthplace? Hard to say.
Your neurons: surely they're an important part of your brain. How about glial cells? They keep your neurons happy — but then, so do your heart and lungs, and for some reason you felt the need to cut them out of the picture. Hmm....
What if you have brain cancer? Perhaps you've had a benign tumor for years — one that's been giving you energy and making you happy. Would you want it removed?
What about your gut brain — the rather large network of neurons living along your digestive tract? If you identify with the neurons in your skull, you might as well identify with all your neurons. But then why stop at the neurons? In fact, don't a lot of cognitive processes, especially your emotions, require other parts of the body in order to function properly?
If 'you' had a body-ectomy, leaving just a brain in a vat, you'd better hope the mad scientist taking care of you decides to simulate the rest of your body, or you're liable to feel very weird indeed — in other words, not entirely yourself.
Beyond the body
So regardless of whether you choose to identify with your entire body or just your brain, it's not at all clear which subset of the things inside your skin are 'you' and which are 'other.'
But it gets even harder. Why can't your body extend outside your skin? Who decided your epidermis was the official boundary? (And besides, aren't a lot of those outermost cells already dead?)
Your skin leaks odors and pheromones: are they not 'you'? Your hair traps a thin cushion of air to serve as a heat buffer. Why is hair-trapped oxygen excluded from the body, while hemoglobin-trapped oxygen gets to enjoy the full benefits of corporeal citizenship?
The next time you're walking down the street, try imagining your body heat as part of 'you'. A stranger whooshes past, brushing away some of your heat but without touching your skin. What's the difference between that kind of 'contact' and official (skin-to-skin or cloth-to-cloth) contact?
If you care about your limbs and your heart, identify them as part of yourself, then you could just as easily identify with an artificial limb or a pacemaker. By why do these things have to be inside or attached to your body? Could you not equally identify with your favorite jacket, your guitar, or your smartphone?
If your body is so porous, why not identify with your possessions? Wouldn't you rather scrape your knee than see your childhood teddy bear torn to shreds? Might you not sooner lose a kidney than your wedding ring?
If you can 'disown' an internal enemy, like cancer, why can't you 'annex' the friendlier parts of the external world?
Speaking of friends: Why not allow yourself to identify with other people — to accept 'them' as just as much a part of 'you' as your limbs? Why do you deny your friends and family the status of selfhood? Are they not a part of you, in a way that's just as important as any tissue in your body? If you can muster some identity for the symbionts (like mitochondria) living inside your skin, why not save some for the symbionts living outside?
Elizabeth Stone gets it. Here she is, on the decision to have a child:
It is to decide forever to have your heart go walking around outside your body.
It helps that we're programmed to identify strongly with our children. But why not let your sense of self extend to encompass much larger parts of the world? Who's to stop you?
Dawkins argues that organisms should be understood as having an extended phenotype, encompassing more than just their bodies — the clam's shell or the beaver's dam, for instance. Surely, then, the same principle must allow for an extended self.
In behavioral terms, you're already doing this — treating the 'external' world like part of your self. You're possessive of your things, jealous of your lovers, protective of your family, and territorial of your space — just like every other member of the human species.
In all of this we've been discussing the spatial boundaries of your self/body. What about its temporal boundaries? When did you begin? And when will you end?
Perhaps you think that you — your body — began at conception, when the sperm hooked up with the egg. But why not earlier? Alan Watts suggests 'you' might have been present in "the evil gleam in your father's eye when he was coming after your mother."
Similarly, when will 'you' end? Is death a precise moment, or do you wind down slowly, sputtering toward a final halt? Does the brain flicker for a few fleeting moments after the heart and lungs have stopped? If you can be revived, are you really dead?
Do 'you' even end at death? Your material body persists — but without the right pattern of energy, the animating spark, there's little to recommend it. The atoms that once danced to the tune of 'you,' now dance to other forces. But what about your legacy, all the things you made and touched? Your children and friends, your life's work, the company you built that lives on with its own élan vital? Do 'you' not remain, faintly, in those scattered wisps of your extended self?
All this leads to the inescapable conclusion: There are no meaningful, objective boundaries to your physical body. No easy answer to cling to, about where 'you' leave off and the rest of the world begins.
What the body really is
None of this should be a surprise, really.
Anyone who thinks about the world scientifically must accept that it's far too complex, messy, and nuanced to be captured by something as crude as human language, as simple as mere words.
Nothing in the real world has clear boundaries. All 'species' are artificial constructs. Some mammals lay eggs! Birds are dinosaurs!(?) Even planets are hard to define. Why should 'you' or 'your body' be any different?
'You' were never just a piece of matter, a mere collection of atoms. Matter comes and goes. Atoms cycle through your system, cells die and new ones take their place — and no one seems to care. On the other hand, simply rearrange your matter — by stuffing yourself through a meat grinder, say — and although your atoms will persist, suddenly 'you' will be gone.
Maybe if your body were a hard, fixed object, like an aluminum can(*), things would be different. But it's not. Your body is a streaming pattern of energy, more fluid than solid. As Heraclitus noted some 25 centuries ago, you can't step in the same river twice, because neither you nor the river are fixed entities. Like the river, your body flows through space and time. Like a flame, it metabolizes matter and energy, attracting a certain set of atoms, for a time, then releasing them back into the world.
(*) Actually an aluminum can is more fluid, more like a flame, than you might realize. Exposed to the air, aluminum will slowly rust, via the chemical reaction known as oxidation, the same reaction behind a burning flame. Every piece of aluminum you've ever seen, then, has been burning — just really, really slowly.
Your body is like a whirlpool or a tornado — an eddy in the universe. Something kicks the eddy into motion, it swirls around for a time, and then dissipates. While it's swirling, you can point in its general direction, but you can't isolate it, put it in a box, or delineate its boundaries.
This molecule over here: is it 'in' or 'out' of the eddy? Hard to say. If it's swirling in the vortex, right this very moment, we're likely to call it 'in.' But even if it's hundreds of yards away, who's to say it's not part of the production (though not yet on the main stage)? Who's to say it's not faintly in the eddy's thrall, slowly being coaxed into the dance?