When Animal Liberation was published, I hoped that, 40 years on, there would be no more slaughterhouses – and therefore no more newspaper stories about atrocities like the one at an abattoir in the north of England. The arguments against our oppression of animals seemed to me so clear and irrefutable that surely a powerful movement would arise, consigning these abuses to history, as the anti-slavery movement had put an end to the African slave trade.

At least, that is what I thought in my more optimistic (or naive) moments. In my more pessimistic (or realistic) moments, I understood the vastness of the task of changing habits as deeply ingrained as eating meat, and transforming philosophical outlooks as fundamental as speciesism. More than 200 years after the abolition of the slave trade, racism is still with us, and even slavery, though everywhere illegal, still exists. How could I expect ending speciesism and animal slavery to be easier or more swift than ending racism and human slavery?

Against the background of those more realistic assumptions we can deplore the fact that animals are still being mistreated on a vast scale, but we should not despair. In many parts of the world, including Europe and the US, there has been tremendous progress in changing attitudes to animals. A powerful animal advocacy movement has emerged, and it has made a difference for billions of animals.

In 1971, when a few other students and I set up a display in Oxford to show passers-by how their eggs and veal were produced, people asked if we really imagined that we could win against the political and financial might of the agribusiness industry. But the animal movement has challenged that industry with success, achieving reforms across the entire European Union that require farm animals to have more space and better living conditions, and similar changes have now become law in California as well. Admittedly, these changes are still far from giving factory-farmed animals decent lives, but they are a significant improvement on what was standard practice before the reforms came into effect.

Perhaps even more satisfying is the number of people who have abandoned eating animals entirely, and the others who have cut down their meat consumption for ethical reasons. In the 1970s, to be a vegetarian was to be a crank – a thought reflected in the self-mocking name of what was then London’s best vegetarian restaurant, Cranks. If you used the term “vegan” you invariably got a blank look and had to explain what it meant.

Despite all this, it is probably still true that there are more animals suffering at the hands of humans now than ever before. That is because there are more affluent people in the world than ever before, and satisfying their demand for meat has meant a vast expansion of factory farming, especially in China. But to see this as an indication that animal advocates have made no progress would be like saying that because there are more slaves in the world now than there were in 1800, the anti-slavery movement has made no progress. With the world’s population now more than seven times what it was in 1800, numbers do not tell the whole story.

Progress is not steady. There will always be periods in which we seem to be treading water, or even going backwards. Periodically articles appear about the resurgence of fur, for example, but I doubt that fur will ever be as uncontroversially accepted as it was 40 years ago. The fact that newspapers give extensive coverage to stories about the abuse of animals being slaughtered for food (not only about abused dogs, cats or horses) is itself a sign of progress.

Meanwhile, there is a simple lesson to draw from the videos released by Animal Aid investigators: if you turn animals into things to use, and give workers complete control over them, it will never be possible to stop the occurrence of the kind of abuse allegedly shown in the videos. Sacking one or two workers merely makes a scapegoat out of them. (Think about what that word tells us about our traditional attitude to animals.) The problem is not one or two workers, nor the practice of halal slaughter, but the system, and the system will not change until people stop buying meat.