David Aaronovitch

It was not Laurence Cottam's finest hour. With one blow, the 56- year-old man had knocked another chap - Joe Wilkinson, a complete stranger - into the Mr Kipling cake display at Morrison's department store in Wakefield. On Tuesday, this act of violence cost him nearly a thousand quid in fines and compensation at Wakefield Magistrate's Court.

But why did he do it? How had Mr Wilkinson so offended him that it merited the expense of so much physical effort, money and, of course, reputation? What obscure rule had been broken, or insult rendered? Had Mrs Cottam been called the daughter of a mangy camel, perhaps? Or was there a Verona- style family feud between the houses of Cottam and Wilkinson?

It will not surprise regular visitors to our modern supermarkets that the answer to this riddle involves a shopping trolley and a revolving door - an almost unbeatable combination of resistible force and movable object. Laurence and his wife, Sylvia, were negotiating the doors with their own trolley, when Joe (who was behind them) misjudged his speed. Imagine Sylvia's little bark of pain, the hurt expression she turns on Wilkinson, her words of reproof - and her husband's feeling of protective anger. Joe apologises and goes on his way.

Laurence is not mollified by the expression of regret. In Wakefield, as in the mountains of Albania, honour apparently demands blood sacrifice, so he follows Joe and, at the first available opportunity, rams his own trolley into his adversary's ankles. Words follow, until, surrounded by the gaudy sweetness of the cake department, Laurence's fist flies (appropriately) into Joe's cakehole. We can only feel glad for Mr Wilkinson that the denouement did not take place over the wet fish counter.

We need now to transcend the obvious. It is clear that Mr Cottam is a man involved in a losing battle with his own id, and that Sylvia would be well-advised to do the shopping on her own. It is also true that acts of macho stupidity can disfigure almost any activity at any place. But it does appear that there are special and growing problems in supermarkets, involving fights in check-out queues, trolley-barging and bottles of wine broken, Western saloon style, over heads.

Reflecting upon this, and upon my own experiences in Safeways and Sainsburys, I am beginning to think that what we are witnessing is not the breakdown of civility, but a state of pre-civility. Consider. Vast areas of our public lives are regulated by a million unstated little rules, which we all come to understand governing matters to do with precedence and personal space.

They are not laws; they are not even bylaws. Once, women like Emily Post would write them down, but this is no longer done, and today we do not even pass them on verbally. So, built over time like the British Constitution, they are simply absorbed.

In a cinema, for example, you may talk through the commercials, and through the title music of the main picture, providing that there is no action taking place. You may place your empty ice-cream carton carefully under your chair, but not in the aisle. If you find your seat covered in the coats and accessories of the person next door, you will stand there until they remove them (usually apologetically) themselves.

But supermarkets are too new for us to have evolved such clear and well- understood rules. They are the new frontier, and in them we all become pioneers, bringing with us our own rough notions of justice. How long may you stand in front of the baked beans consulting your shopping list, but barring access to others? If your unattended trolley is blocking egress, can it be moved by someone else? If so, how far can it be moved without violating rules of ownership? Is priority given to those emerging from aisles, or to those in the corridors at either end? Only at the deli counter is order codified, and even then, we do not know whether the needs of those wishing to examine what lies under the glass take precedence over those waiting to be served.

It follows, therefore, that it is still almost impossible to calibrate the social seriousness of having your wife's shins attacked by a strange trolley, let alone the necessary admonitory response. So Mr Cottam's jab in the kisser is not to be seen simply as an act of violence, but should also be interpreted as a plea for guidance.