Round about this time last year BP published a full-page advertisement in the Spectator that proclaimed the message, "Thanks to BP you can now see a good clean fight at the Tate."

This was not, alas, a reference to any modern in-fighting at the gallery or a description of Brian Sewell waltzing into battle against the Modernists. Half of the ad was devoted to the reproduction of a large oil painting by Johan Zoffany, called Colonel Mordaunt's Cock Match. This little-known but attractive-looking canvas was painted in India two centuries ago, and shows a few British army officers and a lot of Indians preparing to watch two fighting cocks battle to the death, the gist of the advertisement being that good old BP had helped the Tate to clean up this latest acquisition, though not to buy it.

What sticks in my mind, however, is not so much the painting itself as the description of the painting offered by the copywriter. "Although by today's standards it's a cruel subject, emerging from years of grime is a brilliantly colourful, vivacious document of a great 18-century social occasion."

I was brought up short by that description of the painting. Not the grime bit. (Is a bit of dirt on a painting anything compared to an ocean-going oil slick?) Nor even by the startling idea that a cockfight run by some army officers way out in India was a great 18th-century social occasion.

No, I was brought up sharp by the idea that the painting had, by today's standards, a cruel subject. Yes, there are two cockerels in the foreground waiting to cause maximum damage to each other. Yes, a lot of people are standing around waiting to see the result. But surely not even an over- careful copywriter, or perhaps a faint-hearted director of BP, can say that by today's standards this is a cruel subject.

Even in an army context one can think of more cruel things. A painting by a modern Johan Zoffany entitled Three Drunken British Soldiers Raping and Killing a Girl on Cyprus might strike one as a more cruel idea. And every night on the news there are much more horrific things reported, whether it be massacres in former Yugoslavia, or Dunblane, or murders in America, or road deaths in Britain.

But keeping it just to animals, the prospect of slaughtering half the cows in Britain, whose only crime was to have been born, is not one that I find easy to contemplate, especially when people like Stephen Dorrell say that the important aim is not to clean up the food chain but to restore consumer confidence. Let's cheer up the public - let's slaughter half a million cows! In human, and Eastertide, terms, that's a bit like Pontius Pilate justifying crucifixions by saying that a bit of selective culling would raise public confidence.

In 1784 they set two cocks on each other, and BP is still apologising; in 1996 we turn a blind eye to, for instance, the horrible concentration camps known as chicken battery farms that dot our countryside behind barbed wire and "Keep Out" notices, and for which nobody apologises at all. People sometimes get hot under the collar if you suggest that there is any comparison between German concentration camps and modern battery farms, but I disagree. I think the same dulling of sensitivity is involved, the same kind of inhumanity in being able to close your eyes to suffering.

Still, cruelty strikes different people different ways. Some people devote their leisure time to sabotaging fox-hunting, or trying to ban boxing. Others go to Madame Tussaud's. I was once taken there as a child. The horrific images of torture on offer in the Chamber of Horrors were enough to give me nightmares, and put me off violence and cruelty for life, even though many of those tortures were invented by one religion to deal with another one. In fact, I realise now that many of the images of cruelty I retain from my youth come from Christian sources - all those stonings, and burnings, and massacres, and hideous deaths from Foxe's Book of Martyrs - and I can understand why many non-Christians sincerely find the sign of the crucifix a strangely cruel symbol for a major religion.

Or to put it another way, if BP had paid for the cleaning and restoration of a painting of Christ on the cross, I wonder if it would have occurred to them to apologise for what, by today's standards, seems a cruel subject?