Recently on NPR, Lulu Miller did a piece on a man named Martin Pistorius who fell ill as a child due to cryptococcal meningitis. With all of his motor function lost, Martin eventually went into a coma and while his parents took care of him, the doctors' assessment that Martin was essentially a vegetable left them with no hope of recovery and only the task of waiting for Martin's eventual death. However, weeks turned into months turned into years--Martin would not die. Moreover, as we would all eventually find out when Martin recounted his story, he actually awoke to consciousness but not yet able to move any muscles and so communicate by any means. Martin was alive, mentally, consciously, though without any external indications. To get the full story, you can buy his book here.
The main issue I want to highlight is that what would have appeared to so many as a typical case whose solution is euthanasia--a human person who no longer showed any active brain functioning--was in fact not dead and though he fought through depression after regaining consciousness, returned to live a most fruitful, happy, and productive life on terms which even Peter Singer could live with.
It begs the question both in terms of lack of consent on the part of the person who is paralyzed in such a fashion as Martin was, whether the lack of external indications is sufficient for determining the absence of a functioning mind. The sanctity of life demands that we not execute fatal measures when we are unable to determine whether someone is alive. It seems clear enough that doctors know less than they thought they did when it comes to that dynamic force which animates our mortal coil.