THERE is an old joke about an economist marooned on a desert island with a small group of scientists, a large pile of tinned food and no can opener. The chemist suggests one way of opening the cans, the physicist another. Neither plan is viable as the economist is quick to point out. ''Okay then,'' says the chemist, ''what's your plan?'' The economist produces his overheads and laser pointer, and begins his presentation: ''Let's hypothesise that we have a tin opener . . .'' Let us hypothesise that China and the UK suddenly resolve their differences. We could then privatise the MTR, as it is badly in need of privatisation. It is a beautifully run, modern and well-capitalised public utility and the people of Hong Kong deserve to own it. Sir Hamish Macleod said last night that the current expansion programme and airport-related expenditure made dividend payments a far distant prospect. Therefore, privatisation was a non-starter. But I beg to differ. The initial public offer would be huge. A large tranche of shares would be floated in New York; another in London. Special shares would be offered in Hong Kong, which could be targeted at the small investor in the way that hugely successful British privatisation offers were. The privatisation of British Telecom put a major public utility into the hands of millions of small shareholders in the UK - many of whom had never invested in shares before. Part of the proceeds of the MTR privatisation could go to paying off debts and the Government could also chip in. With debt repayments taking a smaller chunk out of the company's revenue - currently about $650 million from about $2 billion in total revenues - the MTR would be a killer blue chip in any portfolio. And if the thing was done properly, every small Hong Kong investor could benefit. Now, let's suppose they strike gold on Chek Lap Kok.