What must it be like, to live with a brilliant mind trapped inside a rapidly deteriorating body?
British academic Tony Judt is one of our greatest living historians, courageous, controversial and passionate.
He has castigated French intellectuals and politicians of the post-war era for their "self-imposed moral amnesia" concerning collaboration with the Nazis and drawn the fire of many Zionists for his views on Israel.
His 2005 book, Post-war: A History of Europe Since 1945 won huge praise (and some criticism) for its sweeping, encyclopaedic scope and challenging conclusions.
Today he is fighting his toughest yet, racing against time and a failing body to finish his life's work.
In 2008, shortly after recovering from cancer, Judt was diagnosed with amyotrophic lateral sclerosis or ALS, a progressive, fatal, neurodegenerative disease also known as Lou Gehrig's Disease.
Since October 2009, Judt (who is 62) has been paralysed from the neck down.
He has spoken and written with great honesty about his life with ALS, most notably in a series of short columns in The New York Review of Books.
Judt has told how he is now completely dependent on nurses and assistants to live and work in his home in New York and how he has watched his body effectively shut down.
"What is distinctive about ALS -- the least common of this family of neuro-muscular illnesses -- is firstly that there is no loss of sensation (a mixed blessing) and secondly that there is no pain," he says.
"In contrast to almost every other serious or deadly disease, one is thus left free to contemplate at leisure and in minimal discomfort the catastrophic progress of one's own deterioration.
"In effect, ALS constitutes progressive imprisonment without parole".
The historian has not yet entered the stage of the disease where he will require mechanical help to eat and breathe.
But he does depend on his wife (they have two young boys aged 15 and 12) and his nurses and assistants to help him with every aspect of his daily life.
"To say the least, I am utterly and completely dependent upon the kindness of strangers (and anyone else)," he says.
Judt is still working, he can dictate to assistants but even this requires huge effort and great discomfort on his part, as he told BBC Radio 4 recently.
"I've been told, for example, that I'm supposed to consume 3,000 calories a day even though I don't have a single moving muscle below my neck, because the sheer effort of talking alone consumes as much as a professional runner when you have a body in this condition."
"So your choice is to lie very bored, very vegetative, for a very long time, or else to say "sod it, I intend to do something. Well, what'll I do?"
"If I'd been a plumber, it would be catastrophic. But the thing I have done well all my life is read, write, talk, think, teach, disagree, explain and so on, and I can still do those things.
"So I've made a quite conscious decision, that I would do those things as long as I could, even if they shortened my life by tiring me out, which they may do. I don't know if that's true or not, but they may do.
"And since that decision, I've written the equivalent of two and a half books, given a public lecture, taught graduate courses, I've dictated probably thousands of emails. And I feel good. So it works."
Judt does not know how long he has left or how quickly his disease will progress.
The theoretical physicist Stephen Hawking was first diagnosed with ALS in his early twenties and continues to work into his late sixties.
However, ALS can progress at different rates and Judt, at 62 years of age, has deteriorated relatively rapidly since the initial diagnosis.
His race against time to complete his life's work is mirrored in the experience of another well-known, 62-year-old English author, albeit a writer working in a very different field.
Terry Pratchett, the much-loved creator of the multi-million selling Discworld comic-fantasy novels (he has sold 45 million books to date), was diagnosed with early-onset Alzheimer's disease in December 2007.
Pratchett has since written three best-selling novels, talked extensively about his experiences with Alzheimer's and donated more than a million dollars for research into the disease.
In a typically funny, very human speech to the Alzheimer's Research Trust Conference in the UK in late 2008, Pratchett spoke about his reaction to the diagnosis and his plans for the future.
'Apparently I reacted to this situation in a reasonably typical way, with a sense of loss and abandonment with an incoherent, or perhaps I should say, violently coherent fury that made the Miltonic Lucifer's rage against Heaven seem a bit miffed by comparison," he said.
"That fire still burns.
"I want to go on writing! Admittedly, that means I have to stay alive.
"You can't write books when you are dead, unless your name is L Ron Hubbard.
"And so now I'm a game for real. It's a nasty disease, surrounded by shadows and small, largely unseen tragedies".
Pratchett has continued to work and campaign for greater funding for research and better services for patients and their families.
Diagnosed with Alzheimer's at 59, he was told by the British National Health Service that he was too young to qualify for free medication.
"The NHS kindly allows me to buy my own Aricept because I'm too young to have Alzheimer's for free, a situation I'm okay with, in a want-to-kick-a-politician-in-the-teeth-kind of way," he told the conference.
Recently knighted, Sir Terry Pratchett has just been named Published Writer of the Year by the Brit Writer's Awards for his new book Nation.
Judt and Pratchett may work in two very different worlds.
But they are both remarkable men who are determined to rage against the dying of the light.