This year's Nobel Peace Prize winner Liu Xiaobo is serving an 11-year prison sentence at a jail in north-eastern China.
Largely cut off from the outside world, the 54-year-old spends his days reading, exercising and writing letters to his wife.
His crime was to suggest that China should begin peaceful political reforms.
But not everyone who proposes political change ends up jail.
An open letter was recently sent to the government by 23 mostly Communist Party elders calling for more freedom of speech.
In China that is potentially a dangerous thing to do, but the letter was tolerated.
Why is one man put in prison but others are allowed to speak freely?
"Dissent is largely perceived as whether it's a threat to the Communist Party's monopoly on power," said Professor Joseph Cheng, of Hong Kong's City University.
"If it threatens the party, it will be attacked. But if it's seen as acceptable then it is usually tolerated."
There are no political parties in China that oppose the government; even campaign groups and non-governmental organisations are closely monitored to make sure they do not become political.
Independent trade unions are snuffed out before they are established.
But there is dissent: every year there are thousands of what the government calls "mass incidents", demonstrations that are often motivated by a single grievance.
Sometimes they turn violent, but most are quickly contained.
There are also individuals who dare to speak out on bigger issues, despite the fact that doing so often brings a reaction from the government.
The human rights lawyer Gao Zhisheng was one such person.
This self-taught lawyer was once praised by the government, but officials turned on him when he started defending people who criticised them.
He also got involved in politics, writing an open letter to the US Congress that detailed human rights violations in China.
Mr Gao's law practice was closed down and four years ago he was given a suspended prison sentence for "inciting subversion".
Then in February 2009 he disappeared and, despite a brief re-appearance earlier this year, he has been missing ever since.
The lawyer's older brother, Gao Zhiyi, continues to search for him.
The elder Mr Gao is a farmer from Shaanxi province and knows little about what his brother has done - but he is motivated by a sense of justice.
"What crime has he committed? What law has he broken? Why does the state detain him like this?" he said on his most recent visit to Beijing a couple of weeks ago.
"I can accept it if my brother has broken the law and is in prison. But no one will tell me where he is," he added, just before boarding a long-distance bus for the 10-hour journey back to his village.
Gao Zhiyi is poor, so could spend only four fruitless days in Beijing, asking questions about his brother at government departments and police stations.
He promises to come back when he can because he believes China should not be a place where people can simply disappear.
Freedom of speech
Others agree. Liu Xiaobo helped draft a document called Charter 08, published to mark the 60th anniversary of the adoption of the Universal Declaration of Human Rights.
Charter 08's aim is to set up a fairer political system - a separation of powers, legislative democracy - that would prevent human rights abuses.
But Mr Liu's blueprint, if initiated, would mean a fundamental change to China's political system.
That is why he was arrested, charged and imprisoned according to Professor Cheng.
Party elder Xiao Mo says he has come under no pressure from the government since signing the open letter to the government calling for more freedom of speech.
The 74-year-old is happy to explain his views to anyone who turns up at his small, book-filled apartment in Beijing.
It is better to try to change the system from within, he explains, than seek its destruction from the outside.
"Liu Xiaobo's ideas will never work in China. They are not realistic," said Mr Xiao, the former head of the architecture research centre at the Chinese National Academy of Art.
"He wants to do it all in just a single step. That's impossible - and dangerous. If we did what he wanted, there would be chaos."
One great fear that haunts China's communist leaders is that the country will collapse just as the Soviet Union did in the 1990s if it initiates reforms.
But it is difficult to believe that China will not undergo some kind of political change over the coming decades.
Liu Xiaobo and Xiao Mo both want it - and so, apparently, does the government.
Chinese Premier Wen Jiabao has talked on a number of occasions recently about the need for political reforms to underpin the economic transformation.
In an interview with a US magazine in October, he said: "I believe freedom of speech is indispensable for any country."
At the recent annual meeting of China's top communist party leaders they announced that they would pursue "vigorous yet steady" political change.
But the evidence suggests that any reform will come slowly.
And those who want to express their dissent - or just their opinions - will have to continue treading carefully.