When I heard on the radio that Velázquez's portrait of his mulatto assistant had been sold for £2,310,000 I burst out laughing. It was the extremity which counted, like an outrageous joke cracked in the throne room. Normally I forget quickly about such transactions, mindful of Oscar Wilde's gentleman who knew "the price of everything and the value of nothing". The saleroom's connections with aesthetics are too tenuous to be bothered about.

This time, though, I had the sensation that my laugh concealed some other kind of shock, and slowly a feeling of indignation took over. I felt as though I had witnessed a public indignity. A great painter had been affronted and a great painting (to judge by reproductions) defaced. Never again can this honest, serious record of a friend be looked at simply as a human communication, a work of art. It has been consigned to the cabinet of curiosities, to be stared at, quoted in the book of records and cited in the price indices. In the future it will bear its price-tag like a scar.

In one way this kind of sale is artistically irrelevant (except perhaps that it makes art irrelevant to us). The exceptional price is a normal result of demand and supply – Velázquez was no great producer – and of no more moment than other "record prices" which almost daily make the headlines.

But something else is involved here. Artists today, particularly young artists, are worried about the fact that objects which start out as sincere personal statements about life or love to society are turned, by the fatal Midas touch of capitalism, into gold. They become mere units of wealth, to be bought and sold in a cattle market which makes the Miss World competition look like a peace prize award, hoarded against inflation, salted down for lean years on the stock death duties or, at best, hung around some bloated salon as symbols of conspicuous culture.

This spectre – the spectre of genuine creation wrung from the artist's doubts through sweat and suffering being transformed into an objet d'art to be tossed from saleroom to saleroom – is what haunts many sensitive young artists today. It is what drives them to think up every conceivable way of cheating the auctioneer and his accomplices, the public and private collector.

They will carve their sculpture in ice, draw on sand, float monuments out to sea, devalue their own products by reproducing them as multiples – anything to escape from the dealer. Obsolescence becomes equated with human values, eternity means death.

In this context an egregious piece of art salesmanship like the marketing of the Velázquez stands as a sign of everything young artists detest, and must be accepted as a serious blow to all those who deal in the traditional framed canvas. The supply is likely to decline and the whole concept of a painting as a pure spiritual exercise to be badly soiled. It is not only money which is devalued here, art too is diminished.

It is only fair to keep cool about this case. Ten paintings at a tenth of the price would have amounted to the same rake-off for the auctioneer (10 per cent) and the same bonus to insurers, security men, art journalists and all the other beneficiaries who milk off side profits – such valuables secrete wealth like pus. It is just that this notable transaction epitomises a serious trend.