Does the chief executive only care about the smog if it keeps away tourists and foreign businesses? Does he not care for the health of those who live here? Does he really believe that it is only our view, not our health, that is at stake? Or is it just a way of sidestepping the risk of offending the vested interests who support him politically - and give cosy jobs to ex-civil servants?

Either way, it is sad, but the one thing we do not want to do is put him in jail, let alone execute him, for denying 'the truth'. That is something of which the Catholic Church and the Communist Party have had plenty of experience. Even they may have learned lessons that fear is only effective in the short term in spreading a gospel.

Let the facts speak for themselves, and they will if they are repeated often enough by a sufficiently large proportion of society in general, as well as the scientific community.

Letting facts speak for themselves seems rather difficult at present for two great nations - France and Turkey - who ought to know better. Even more remarkable is that they have become embroiled in a dispute not over what they did to each other but an issue of marginal interest to France: the massacre of Armenians in Turkey in 1915. This is no China-Japan dispute over the Nanking Massacre or the Yasukuni Shrine.

Turkey has some laws, allegedly in the interests of national solidarity, which make it illegal to denigrate the nation, and the founder of modern, secular Turkey, Mustafa Kemal. This makes honest appraisal of the events of 1915 difficult. Turkish bone-headedness on this topic is an obstacle to joining the European Union. (These laws remind me of threats in Hong Kong to outlaw suggestions that Taiwan is not part of China, history notwithstanding).

But at least the Turks are just silencing each other. Now the French parliament has passed a law making it an offence to deny that the 1915 killings were 'genocide'.

Certainly, probably hundreds of thousands of (Christian) Armenians in Turkey were massacred. But to use the word genocide is dubious. Most of the evidence points not to government-directed massacres but to spontaneous, communally led killings.

At the time, Turkey was being invaded in the west by France and Britain (at Gallipoli) and (more successfully) by Russia in the east. The Russians used Christianity in an effort to detach Armenians in eastern Turkey to their cause - with some success. So the response of the Muslim Turks wasn't surprising. But don't expect the French parliamentarians to bother with such facts, or to look at whether the French record of massacre in Algeria also counts as genocide.

Of course there are historical episodes whose basic facts are beyond dispute. The Holocaust is one. But should those who deny it be jailed? Either they are ignorant to the point that no one will take them seriously. Or they have an agenda which can only be served by martyrdom.

Constructing an over-arching truth from a series of facts is difficult. But facts are facts, and sooner or later they penetrate most skulls, just as those particles penetrate the lungs as surely as they are seen by the eyes.

Philip Bowring is a Hong Kong-based journalist and commentator