He can't really eat. He can't swallow, unless someone puts a spoon on his tongue to make his throat contract. He can't scratch an itch.
But he can cry.
Tony Nicklinson can't control the muscles in his face, or the way it sometimes turns into a mask of grief, or wipe the strings of saliva that sometimes hang from his lips, but his tear ducts seem to be working just fine.
Before the headache that hit him in Athens seven years ago, everything in his body worked fine. Now he is carried in a hoist from a bed to a chair.
"This is my home," he said to the TV cameras on Channel 4's Dispatches on Monday. "It's also," he said, "my prison." The headache in Athens was a stroke. The doctor who saw him didn't think he'd survive. Now he wishes he hadn't.
The trouble is that Tony Nicklinson can't die. He can't - unless he decides to starve himself, which might take months - end his life unless he gets someone else to help. Which, under British law, would be murder.
It's an issue that he, or rather his lawyer, raised at the High Court in London on Tuesday.
He wasn't seeking, he told the judges, to "introduce an all-encompassing new regime legalising euthanasia and assisted suicide". What he wanted, he said, was for the court to allow "the common law defence of necessity" to a doctor who helped him die.
What he wanted, in other words, was for us to answer a question that almost nobody can answer. Most of us - moving, talking, laughing people - don't know what it's like to feel that you want to end your life, but are unable to. And most of us would look at the tears trickling down Tony Nicklinson's gaunt cheeks, at the wife and daughters who want what he wants, and feel that we want it, too.
But the law isn't about how we feel. The law isn't about how you feel if you were once healthy and fit and happy and now aren't.
The law, as Lord Falconer said on that Dispatches programme, is the same for everybody. "If people want to kill themselves," he said, it's an "entirely private matter", but "they can't kill somebody else".
The law, as the disability rights campaigner Kevin Fitzpatrick also said on the programme, is meant to offer protection. "When you develop a society where some people judge that other people's lives are not worth living," he said, "that's the Rubicon". If you're disabled and have decided that you don't want to live, but that the only way to end your life is by starving, then it's very, very sad for you. It's sad and hard and frustrating and unfair.
But quite a few things in life are sad and hard and frustrating and unfair.
And if the law that makes you sad makes most people safer, then maybe your sadness and the sadness of the people who love you is the price we all have to pay.
And maybe, if you're sad and frustrated and angry, and tell newscasters that you never look at your family and think you can't bear to leave them, because you can't bear to "tolerate so many indignities", you might think about the tweet you sent the 26,834 followers you gained since joining Twitter last Sunday.
"What joy," it said, "it is to be loved." And maybe you'll look at that word 'joy' and think that the upside of being alive is that you can still change your mind.