At some point in life, you've probably asked yourself this question. Or perhaps, you've had to deal with it first-hand.
If someone in your family had a severe brain injury and was in an uncommunicative state, what would you do?
Keep them on life support? Pull the plug?
It's one of the most difficult and complicated decisions any of us could make, because we don't know if they're actually alive "in there".
Well, here's a story that makes things even more complicated.
A London, Ontario man - who was thought to be in an uncommunicative state for years - has been able to tell scientists he's not in any pain.
Scott Routley was asked questions as researchers examined his brain activity through an fMRI (functional magnetic resonance imaging) machine.
During the scan, researchers say Routley was able to communicate with them through the power of thought.
"Scott has been able to show he has a conscious, thinking mind," said Professor Adrian Owen who led the team at the Brain and Mind Institute, at the University of Western.
"We have scanned him several times and his pattern of brain activity shows he is clearly choosing to answer our questions. We believe he knows who and where he is."
This video shows the moment when Routley told Professor Owen he's not in any pain.
Professor Owen was speaking on a BBC Panorama programme that airs tonight.
This is the first time such a severely brain-damaged patient has been able to give answers that are clinically relevant to their care.
Routley, 39, suffered a severe brain injury in a car accident 12 years ago.
His parents say they always thought he was conscious and could communicate by lifting a thumb or moving his eyes. But doctors never accepted it.
Until now, doctors say - based on physical assessments - he never showed any signs that he was aware of the outside world, aware of himself, or had any ability to communicate.
Professor Owen says this discovery is a game changer, and could change the way we treat severely brain-damaged patients who can't move or speak.
"Asking a patient something important to them has been our aim for many years. In future we could ask what we could do to improve their quality of life. It could be simple things like the entertainment we provide or the times of day they are washed and fed."
Professor Bryan Young, from University Hospital in London, Ontario, has been Routley's neurologist for 10 years.
He says the results of the brain scan change everything, as far as how doctors perceived Routley's condition.
"He had the clinical picture of a typical vegetative patient - no emotional response, no fixation or following with his eyes," said Professor Young.
"He didn't have any spontaneous movements that looked meaningful and I was quite impressed and amazed that he was able to show these cognitive responses with fMRI."
During an fMRI brain scan, Professor Owen's team can produce images of "active" regions of the brain by tracking the flow of oxygen-rich blood.
Patients are asked to imagine playing tennis or walking around their house. The idea is to produce unique patterns of activity in different parts of the brain.
Through the fMRI scanner, researchers can ask yes or no questions. One type of brain activity is taken as a "yes" and the other as a "no".
Professor Owen has previously shown that nearly one in five severely brain-damaged patients may in fact be conscious.
Another Canadian patient, Steven Graham, was asked if he knew about his two-year-old niece, who was born after his car accident five years ago.
Graham answered yes, which shows that he was able to create and store memories.
The BBC's Panorama programme followed several uncommunicative and minimally-conscious patients in Canada and Britain for more than a year.
The programme is called 'The Mind Reader: Unlocking My Voice.' It airs tonight on BBC1 or you can see it later on the BBC iPlayer using the link above.
Here's the trailer.
A few years ago, a Belgian man named Rom Houben made headlines after researchers said he communicated after 23 years in a coma.
He was filmed apparently tapping out messages on a special touchpad keyboard with the help of his speech therapist.
But the same researchers later said the method didn't work, and there was no evidence he could actually communicate.
As the lead doctor put it, "The story of Rom is about the diagnosis of consciousness, not communication."