Science fiction has long played with the idea of projecting unified personalities/minds/"souls" into different bodies. The premise is baked into the plots of stories like Avatar and Caprica. But how would it work in the real world?
That's what the science of "embodied cognition" is all about. The basic idea in this new(ish) research area (which overlaps with cognitive psychology, neuroscience, artificial intelligence, robotics, and others) is this: Your mind is defined by your physical form. Not just in terms of "the mind is what the brain does"-we all are pretty down with that already. This takes it further to encompass the whole enchilada: your mind-your "I"-is a function of a cephalized, bipedal, plantigrade, bilaterally symmetrical body between 1.5 and 2 meters tall with two arms terminating in five-fingered hands with opposable thumbs, two lungs, a warm-blooded vascular system, mostly hairless skin, two front-focused eyes, etc. etc. Change any aspects of that physical configuration-in subtle or radical ways-and the mind will inevitably change too.
That might sound a bit "no shit, Sherlock" at first blush, but it's actually got profound implications about what it means to be recognizably human "on the inside." In fact, that very phrase might not even make sense. After all, there's no "little you" inside your body "looking out", Terminator-style. Your perceptions, actions and thoughts all feel direct, integrated, and grounded. You don't "drive" your body, you ARE it. So why do we still assume that we might take that "little man" out and plop him into a different body to "look out of", without any consequences?
Take Avatar. James Cameron stacked the deck by making the Na'Vi basically humanoid with a few cosmetic changes thrown in. Then again, the Na'Vi are twice as tall as humans, have longer arms, fewer fingers, have bigger, wider-set eyes, and ears that can move independently. Not to mention their prehensile plug-in-brain-tails, which add a major perceptual (and explicitly cognitive, given how the Na'Vi "jack in" to the mass Eywa consciousness) modality that has absolutely no analogue in the human body. What would all of that feel like? What would that think like? (Not human, that's for sure.) The point is that through the lens of embodied cognition, those questions are just two sides to the same coin.
Of course, Avatar doesn't go into any of that interesting territory; it sticks with the "Jake Sully can effortlessly interchange between bodies on the fly" approach. Granted, there is one minor scene showing Jake clumsily acclimating to his Na'Vi body-which makes sense, given that all his proprioperceptive expectations and instincts no longer apply. But it's more likely that the disorientation would be much more profound, and he probably would have needed some serious physical therapy before bounding off into the forest in his hospital gown. His mind, his literal sense of self, would need time to retune and retrofit itself to his new body. And the same thing would happen every time he jacked out of his Avatar body back into his human one. What kind of unpredictable effects would that stress have on his mind, his "I"?
OK-interesting to ponder, but maybe not the stuff of kickass action movies. But still, it does point the way toward a much more interesting angle on Avatar's "going native" plotline. If you spent most of your waking hours embodied as a Na'Vi, how could you NOT be increasingly at risk of going native? Your essential psychological human-ness would inevitably drift and deform-Hell, without some kind of mitigating mechanism, Sigourney Weaver's character should have gone off the reservation long before Sully even showed up. (At the very least, this could have provided a more intriguing reason for how and why Jake went turncoat against his own species so damn fast.)
And what if your "avatar" wasn't something blue and sexy, but much less humanoid? District 9 went a little deeper to this territory in depicting Wikus's metamorphosis into a Prawn. The ideas of embodied cognition also enable a much more bittersweet interpretation of the movie's ending: By the time Christopher comes back to Earth, years later, with the means to fully "re-humanify" Wikus, that very procedure would be meaningless: Wikus will have spent so long embodied as a Prawn that he would no longer BE human "on the inside."
Embodied cognition also gives realistic credence to a scenario like that in Ender's Game, which depicts humanity in an accidental war to the death with a species of insectile "buggers." The buggers are clearly sapient and/or sentient, but utterly, opaquely alien-so much that their most basic assumptions about neutral communication with another species (us) come off as murderously hostile. Situated in an entirely different morphology, emerging from an entirely evolutionary heritage (more akin to stigmergic insects than autonomous mammals), the buggers' "hive mind" was not even recognizable or meaningful to humans as such.
But wait, there's more: Embodied cognition also gives a somewhat ominous real-world context to Orson Scott Card's ingenious "hierarchy of alien-ness" (which he invented to categorize the various intelligences in his Ender saga). If psychology and consciousness are defined by the body, it may very well be that any species we might consider "ramen" (ie, Card's term for "capable of meaningful communication," not instant noodles) would necessarily have to be very, very close to humanoid in physical form-and that the further any species diverges from that, the more likely they are to be "varelse"-intelligent, but completely opaque to communication or human understanding (an extreme example being the sentient planet in Solaris).
In any case, I want to read/see/watch more science fiction that takes these powerful ideas into account. I've seen bits here and there. Asimov's "Bicentennial Man" goes there, sort of, in presenting a robot that becomes more and more "phenomenally human" as his physical body is gradually upgraded to match real human anatomy. And Peter Watts's Rifters trilogy (especially the first book, Starfish) touches on this intelligently as well: the characters are physically modified to be able to breathe seawater at the bottom of the ocean, and, not surprisingly, "going native" (essentially becoming more fishlike, mentally, than human) is an ongoing concern in the plot.