Our concern for the pain and distress of others should be extended to any "painient" - pain-feeling - being regardless of his or her sex, class, race, religion, nationality or species. Indeed, if aliens from outer space turn out to be painient, or if we ever manufacture machines who are painient, then we must widen the moral circle to include them. Painience is the only convincing basis for attributing rights or, indeed, interests to others.

Many other qualities, such as "inherent value", have been suggested. But value cannot exist in the absence of consciousness or potential consciousness. Thus, rocks and rivers and houses have no interests and no rights of their own. This does not mean, of course, that they are not of value to us, and to many other painients, including those who need them as habitats and who would suffer without them.

Many moral principles and ideals have been proposed over the centuries - justice, freedom, equality, brotherhood, for example. But these are mere stepping stones to the ultimate good, which is happiness; and happiness is made easier by freedom from all forms of pain and suffering (using the words "pain" and "suffering" interchangeably). Indeed, if you think about it carefully you can see that the reason why these other ideals are considered important is that people have believed that they are essential to the banishment of suffering. In fact they do sometimes have this result, but not always.

Why emphasise pain and other forms of suffering rather than pleasure and happiness? One answer is that pain is much more powerful than pleasure. Would you not rather avoid an hour's torture than gain an hour's bliss? Pain is the one and only true evil. What, then, about the masochist? The answer is that pain gives him pleasure that is greater than his pain!

One of the important tenets of painism (the name I give to my moral approach) is that we should concentrate upon the individual because it is the individual - not the race, the nation or the species - who does the actual suffering. For this reason, the pains and pleasures of several individuals cannot meaningfully be aggregated, as occurs in utilitarianism and most moral theories. One of the problems with the utilitarian view is that, for example, the sufferings of a gang-rape victim can be justified if the rape gives a greater sum total of pleasure to the rapists. But consciousness, surely, is bounded by the boundaries of the individual. My pain and the pain of others are thus in separate categories; you cannot add or subtract them from each other. They are worlds apart.

Without directly experiencing pains and pleasures they are not really there - we are counting merely their husks. Thus, for example, inflicting 100 units of pain on one individual is, I would argue, far worse than inflicting a single unit of pain on a thousand or a million individuals, even though the total of pain in the latter case is far greater. In any situation we should thus concern ourselves primarily with the pain of the individual who is the maximum sufferer. It does not matter, morally speaking, who or what the maximum sufferer is - whether human, non-human or machine. Pain is pain regardless of its host.

Of course, each species is different in its needs and in its reactions. What is painful for some is not necessarily so for others. So we can treat different species differently, but we should always treat equal suffering equally. In the case of non-humans, we see them mercilessly exploited in factory farms, in laboratories and in the wild. A whale may take 20 minutes to die after being harpooned. A lynx may suffer for a week with her broken leg held in a steel-toothed trap. A battery hen lives all her life unable to even stretch her wings. An animal in a toxicity test, poisoned with a household product, may linger in agony for hours or days before dying.

These are major abuses causing great suffering. Yet they are still justified on the grounds that these painients are not of the same species as ourselves. It is almost as if some people had not heard of Darwin! We treat the other animals not as relatives but as unfeeling things. We would not dream of treating our babies, or mentally handicapped adults, in these ways - yet these humans are sometimes less intelligent and less able to communicate with us than are some exploited nonhumans.

The simple truth is that we exploit the other animals and cause them suffering because we are more powerful than they are. Does this mean that if those aforementioned aliens landed on Earth and turned out to be far more powerful than us we would let them - without argument - chase and kill us for sport, experiment on us or breed us in factory farms, and turn us into tasty humanburgers? Would we accept their explanation that it was perfectly moral for them to do all these things as we were not of their species?

Basically, it boils down to cold logic. If we are going to care about the suffering of other humans then logically we should care about the suffering of non-humans too. It is the heartless exploiter of animals, not the animal protectionist, who is being irrational, showing a sentimental tendency to put his own species on a pedestal. We all, thank goodness, feel a natural spark of sympathy for the sufferings of others. We need to catch that spark and fan it into a fire of rational and universal compassion.

All of this has implications, of course. If we gradually bring non-humans into the same moral and legal circle as ourselves then we will not be able to exploit them as our slaves. Much progress has been made with sensible new European legislation in recent decades, but there is still a very long way to go. Some international recognition of the moral status of animals is long overdue. There are various conservation treaties, but nothing at UN level, for example, that recognises the rights, interests or welfare of the animals themselves. That must, and I believe will, change.

· Dr Richard Ryder was Mellon Professor at Tulane University, New Orleans, and has been chairman of the RSPCA council; he is the author of Painism: A Modern Morality, and his new book, Putting Morality Back into Politics, will be published by Academic Imprint in 2006

ianmerrett@trinkle.fsbusiness.co.uk