A judge in a specialist court has been asked 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.
They say he has limited communication skills but can say "yes" and "no".
But his relatives say he has no "quality of life" and should be allowed to "die naturally".
Mr Justice Mostyn began 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 on Wednesday.
He said he had never heard of the provision of nutrition and hydration being withdrawn from someone who was medically stable.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 the 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.
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 continues.