A PERSON can become preoccupied by one minor social issue, to the point where it fills the landscape and blots out all rival considerations, indeed all sense of proportion. A fine clinical specimen was on display last week, in the form of a reader's letter headlined: ''Smoking's latest victims''. The writer of this piece was one of those people who approves of smoking being banned in aeroplanes. Indeed I think without being unfair to him we can surmise that he approves of smoking being banned anywhere people can be persuaded to ban it. Some people seek to ban smoking because it is dangerous to non-smokers, an understandable if scientifically fragile approach. This writer adopted the alternative approach, wishing to ban smoking because it is dangerous to smokers. This idea is scientifically impregnable, but open to the serious objection that people's selection of personal habits, hazardous or otherwise, is their own business. So here we had David Norris of Wan Chai announcing that Melina Mercouri and John Candy had joined Humphrey Bogart, John Wayne and Bette Davis, ''legendary smokers who paid the ultimate price of their addiction''. Actually this involves a dangerous assumption. ''Smoking-related'' diseases are those which are found more often in smokers than in non-smokers. Smokers as a group tend to die somewhat younger than non-smokers, and it is reasonable to infer that this is a consequence of their greater susceptibility to ''their'' diseases. We cannot, however, jump from there to the conclusion that in every or any individual case, the contraction of a smoking-related disease was a result of smoking. Some people get these diseases without smoking at all. There are a variety of points which could no doubt be made about statistics, the role of chance in human affairs and the possibility that people may knowingly take risks by smoking just as they do if they indulge in sky-diving, pot-holing or British Armyexpeditions to Kota Kinabalu. But the erosion of Mr Norris' statistical faculties is not his most interesting symptom. What I find fascinating is the total disappearance of a sense of taste. After all Humphrey Bogart, John Wayne and Bette Davis were not ''legendary smokers''. They were legendary movie stars who happened to smoke, as many people did in those days. MELINA Mercouri was a splendid actress in her time, and as a politician was a great improvement on most members of that sordid species. I liked her. She deserves better than to have Mr Norris capering and gibbering on her tomb in an effort to draw our attention to the faint whiff of nicotine emanating from it. It is a shame that she had a personal habit which was and is expensive, dirty and rather anti-social. This is the sort of snippet over which it is seemly to cast a discreet veil in obituaries, which is no doubt why nobody but Mr Norris mentioned it. Whether somebody smokes or not, this is not the most important thing about their life. It may have something to do with their death, but it is not the most important thing about that either. In the presence of the last great mystery, there are more important matters to ponder than the chance to make pretty propaganda points about public health policy - or there ought to be. Is Mr Norris going to start turning up at local funerals, carrying a sandwich board drawing attention to what unhealthy personal habits did for the deceased? In my youth it was still customary for gentlemen to stand and remove their hats when a funeral went by, as a mark of respect. This habit seems to have fallen into disuse, either because of a decline in hat wearing or a decline in respect. But things have not gone so far that the socially acceptable responses to a freshly sealed sepulchre include ''serves him right for smoking''. Mr Norris concluded with an invitation to the Tobacco Institute's chief mouthpiece on airline smoking bans to ''comment on the recent departures'' of two ''faithful, long-time users of his products''. This suggestion has not been taken up, so Mr Norris may feel he has scored a point. Perhaps he has. He has also provided a splendid illustration of the way that a devotion to preserving other people's health from their own choices and actions leads gradually, but inevitably, to fanaticism, intemperance and a refusal to respect the humanity of those who are supposedly being helped.