A judge in a specialist court is set to decide whether a bed-bound man with severe learning disabilities should be allowed to starve to death.
Doctors say the man, who is in his 50s, is medically stable and should continue to be fed artificially through a tube.
But his relatives say he has no "quality of life" and should be allowed to "die naturally".
Mr Justice Mostyn is overseeing the dispute at a trial in the Court of Protection, where judges consider issues relating to people who lack the mental capacity to make decisions, in London.
He said he had never heard of the provision of nutrition and hydration being withdrawn from someone who was medically stable.
Lawyers said the case might be the first of its kind.
The judge said it raised a "big question" about how "quality of life" was defined.
Mr Justice Mostyn is analysing evidence from doctors responsible for the man's care and from relatives.
The judge says there is little available evidence about the suffering starvation causes.
He has ruled that neither the man, who lives in England, nor the hospital authority involved can be identified.
Hospital bosses with responsibility for the man's care have asked the judge to make decisions.
Barrister Pravin Fernando, who leads their legal team, has told how the man has a history of severe learning disabilities and had also suffered a stroke and a heart attack.
The judge has also been told that the man has epilepsy.
Medics say has limited communication skills but can say "yes" and "no".
The judge has also been told that the man listens to music on the radio.
Specialists say there is no evidence to suggest that he is near the end of his life.
"At the moment he doesn't seem to be actively dying," one doctor told the court.
"He seems to be medically stable."
The judge has yet to hear evidence from the man's relatives.
But at a preliminary hearing last month the man's sister said relatives agreed that he should be allowed to die.
"He's been in and out of hospital for two to three years," she said.
"He's dependent on the nurses as they do everything for him.
"We just don't feel he should continue to be put through very painful procedures."
Mr Justice Mostyn said the question at the centre of the case was "whether he should be fed at all".
"This could not be a more important issue," said the judge.
"It begs a big question.
"How do you define quality of life? From whose viewpoint? By what standards?"
He added: "I have never heard of withdrawal for somebody who is medically stable before."
The hearing, which started on Wednesday, is expected to end on Thursday.