The great moral philosopher Jeremy Bentham, founder of utilitarianism, famously said,'The question is not, "Can they reason?" nor, "Can they talk?" but rather, "Can they suffer?" Most people get the point, but they treat human pain as especially worrying because they vaguely think it sort of obvious that a species' ability to suffer must be positively correlated with its intellectual capacity. Plants cannot think, and you'd have to be pretty eccentric to believe they can suffer. Plausibly the same might be true of earthworms. But what about cows?

What about dogs? I find it almost impossible to believe that René Descartes, not known as a monster, carried his philosophical belief that only humans have minds to such a confident extreme that he would blithely spreadeagle a live mammal on a board and dissect it. You'd think that, in spite of his philosophical reasoning, he might have given the animal the benefit of the doubt. But he stood in a long tradition of vivisectionists including Galen and Vesalius, and he was followed by William Harvey and many others (See from which this picture is taken).

How could they bear to do it: tie a struggling, screaming mammal down with ropes and dissect its living heart, for example? Presumably they believed what came to be articulated by Descartes: that non-human animals have no soul and feel no pain.

Most of us nowadays believe that dogs and other non-human mammals can feel pain, and no reputable scientist today would follow Descartes' and Harvey's horrific example and dissect a living mammal without anaesthetic. British law, among others, would severely punish them if they did (although invertebrates are not so well protected, not even large-brained octopuses). Nevertheless, most of us seem to assume, without question, that the capacity to feel pain is positively correlated with mental dexterity – with the ability to reason, think, reflect and so on. My purpose here is to question that assumption. I see no reason at all why there should be a positive correlation. Pain feels primal, like the ability to see colour or hear sounds. It feels like the sort of sensation you don't need intellect to experience. Feelings carry no weight in science but, at the very least, shouldn't we give the animals the benefit of the doubt?

Without going into the interesting literature on Animal Suffering (see, for instance, Marian Stamp Dawkins's excellent book of that title, and her forthcoming Rethinking Animals), I can see a Darwinian reason why there might even be be a negative correlation between intellect and susceptibility to pain. I approach this by asking what, in the Darwinian sense, pain is for. It is a warning not to repeat actions that tend to cause bodily harm. Don't stub your toe again, don't tease a snake or sit on a hornet, don't pick up embers however prettily they glow, be careful not to bite your tongue. Plants have no nervous system capable of learning not to repeat damaging actions, which is why we cut live lettuces without compunction.

It is an interesting question, incidentally, why pain has to be so damned painful. Why not equip the brain with the equivalent of a little red flag, painlessly raised to warn, "Don't do that again"? In The Greatest Show on Earth

, I suggested that the brain might be torn between conflicting urges and tempted to 'rebel', perhaps hedonistically, against pursuing the best interests of the individual's genetic fitness, in which case it might need to be whipped agonizingly into line. I'll let that pass and return to my primary question for today: would you expect a positive or a negative correlation between mental ability and ability to feel pain? Most people unthinkingly assume a positive correlation, but why?

Isn't it plausible that a clever species such as our own might need less pain, precisely because we are capable of intelligently working out what is good for us, and what damaging events we should avoid? Isn't it plausible that an unintelligent species might need a massive wallop of pain, to drive home a lesson that we can learn with less powerful inducement?

At very least, I conclude that we have no general reason to think that non-human animals feel pain less acutely than we do, and we should in any case give them the benefit of the doubt. Practices such as branding cattle, castration without anaesthetic, and bullfighting should be treated as morally equivalent to doing the same thing to human beings.

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