By dkl9, written 2025-362, revised 2025-362 (0 revisions)
Suppose your brain is removed from your body, and both are kept intact. It would be sustained in a vat and surrounded by electrodes where it would normally connect to the spinal cord and nerves.
Your body is kept alive. In place of the brain would then be a computer, connected to the spinal cord and nerves as if they were peripherals. The computer in the body connects wirelessly to the machinery around the separated brain.
The whole endeavour far exceeds current technology I've heard of. But if it goes as intended, your brain-in-a-vat would process sensory inputs from the remote body, indirectly and from a distance. Your motor outputs (actions) would still control the body, indirectly and from a distance.
Is that "brainless" body still "your body"? Where are "you"? At the body, moving around in the world, or at the brain, stuck in a vat but controlling and experiencing the body?
In that situation, I would say "you" have two bodies: one that contains the mind, useless on its own, and another that's more useful, but mindless on its own. If you agree with that labelling, that means your body is (or bodies are) defined by function and contiguity of mind, rather than physical contiguity.
From there, you could go on to have more than two bodies. The clearest case might be to grow another brainless human that also connects to the same brain-in-a-vat as earlier. Electrodes on different regions of the vatted brain would lead to signals for different human bodies. If a human brain can adapt to such a situation (it can already adapt to a lot), you would get sensory experience from both remote bodies at once, and control movements of both remote bodies at once, but abstract thoughts would be shared between the bodies in one mind.
If you grow a human arm in a lab machine and affix it in place of your natural arm, fully-functional, surely the artifical arm is part of your body. More realistically, you could instead affix an electromechanical prosthetic. If you connect nerves to wires to make it fully operable, it too would be part of the body. There's nothing special about natural body-matter.
Artifically grow a third arm, and (assuming neurological complications are solved) it too can become part of your body, even if you only ever had two arms naturally. Likewise with prosthetic extra limbs. If you agree with that labelling, that means your body is defined by attachment and function, rather than material or naturalness.
Combine that with the previous conclusion, and not only can you have multiple bodies, but they need not all be biological. The vatted brain could get more electrodes, corresponding to the sensors and actuators of a robot body. Connect the robot to wirelessly obey the vat-machine, and it would become another body for whoever got into this strange situation.
Once we abandon the substrate of a human body, we could just as well abandon the general structure. The remote-controlled robot body need not be humanoid. If you submit to getting your brain vatted — or at least thoroughly BCI'd — you could get a quadcopter body and keep your human body.
If you suppose that outside tools can be part of the brain's mind, then it matters little whether your brain connects to a robot body via electrodes in place of nerves or via a mechanical controller in your hands. The threshold for what counts as an extra body is then much lower than you might have considered before.
A cell-phone call lets you control the speakers on a distant device, and experience what its microphone detects. The interface connects to your brain thru mouth (your voice) and ears. Is the phone on the other end then a secondary body? Intuitively not, but why? You might suppose (I did, at first) this is sith the phone can't walk or otherwise move, but that argument implies paralytics don't have real bodies. But there are more robust reasons:
With current technology, and some effort, you can still get a second body:
Such a setup limits use of your original body. Future BCI might fix that.