I am reading "How Physics Makes Us Free" and have a question about the central Daniel Dennett thought experiment in the opening chapter. The experiment treats body parts, crucially the brain, as a component of the body like a spark plug in a car (brain in a vat). It is, rather, part of an organism and in my mind indivisible from the nervous system. Even when higher brain function is dead a body will still reject a donated organ and attack it as alien. A thousand same-model spark plugs will work in a car without any issues. It is at the level of biology that identity first appears. Yet the thought experiment treats physics and psychology as the only relevant domains. If the thought experiment were true to biology it would not be enough to replicate all the synapses and nerves but the entire body as the biological instantiation of identity. Am I overstating a life-science claim to some part of this scenario?