The chemical formula for sugar is not sweet -- and a philosopher’s ideas about disgust are, by the same principle, not themselves disgusting. On the other hand, it is hard to analyze the experience of disgust without evoking it, or at least coming up with thought experiments that are pretty distasteful. This column is about Colin McGinn’s new book The Meaning of Disgust (Oxford University Press). I mention this in case anyone is eating. You might want to come back when you're done. There will be effluvia.

McGinn, a professor of philosophy at the University of Miami, is a prolific and wide-ranging author, but rather a latecomer to the topic. Quite a bit of work has been done lately in the humanities regarding emotion or affect, variously informed by cognitive research, evolutionary theory, and psychoanalysis. I have no statistics to back this up -- just a rough sense from keeping an eye out over the years -- but it seems as if negative affects receive most of the attention, with shame in particular getting the lion’s share.

The literature on disgust is not quite as abundant, but there’s still plenty of it. The most incisive and stimulating volume on the subject, to my knowledge anyway, is William Ian Miller’s The Anatomy of Disgust (Harvard University Press, 1997), while Martha Nussbaum provides an extensive and judicious assessment of the interdisciplinary literature in Hiding From Humanity: Shame, Disgust, and the Law (Princeton University Press, 2004). Nussbaum’s thinking on the matter -- which she revisited in From Disgust to Humanity: Sexual Orientation and Constitutional Law, published last year by Oxford University Press -- is in part a reply to the work of Leon Kass, a professor of social thought at the University of Chicago and chairman of the President’s Commission on Bioethics during the administration of George W. Bush.

Kass understands disgust to be the product of moral wisdom expressing itself at a visceral level, “beyond reason’s power fully to articulate it.” In the shudder of disgust, human nature “revolts against the excesses of human willfulness, warning us not to transgress what is unspeakably profound.” It is possible to make rational arguments against cannibalism, incest, bestiality, or the mutilating of corpses; but such reasoning lacks the raw suasive power of nausea.

So Kass maintains while making a case against human cloning. “Revulsion is not an argument,” he admits, “and some of yesterday’s repugnances are today calmly accepted -- though, one must add, not always for the better.” Calmly accepting cloning, then, would be a crime against our deeper wisdom. At this point it would seem obligatory to explain how we can distinguish “deep moral wisdom”-type disgust from the old “Klansman shooting interracial couple” sort. All the more so given that Kass played an entirely honorable role in the Civil Rights movement. But he offers no clarification. Disgust is a source of interesting and difficult problems, but it doesn’t offer much of a solution to anything.

Disgust is paradoxical -- “both an aversive and an attractive emotion,” writes McGinn. Either way, it exerts a powerful claim on one’s attention. “Disgust is not boring,” he writes. “It has a kind of negative glamour.”

But how is it that we are susceptible to that dark glamor? Animals seem not to manifest disgust, nor do humans until sometime around age 3. Dogs and very small children are notoriously fascinated with excrement (their own and otherwise). What counts as disgusting can vary from one society to the next. Whether you find escargot, haggis, or pork rinds to be delicious or nauseating is a matter of cultural conditioning. In spite of such local variations, though, the range of kinds of things eliciting disgust seems limited. From era to era and culture to culture, the feeling of disgust tends to focus on some aspect of ingestion, excretion, sexuality, or death -- or some shudder-inducing combination thereof.

Conversely, not just anything can be experienced as disgusting. (People use the word a bit loosely at times, often while angry or frustrated; but real disgust involves revulsion and a sense of possible contamination.) It’s hard to imagine anyone, however squeamish, feeling disgust for a laptop computer or a redwood tree.

We are dealing, then, with an emotion that is somewhat flexible but by no means arbitrary, with a particular emphasis on the flesh -- its weaknesses, its smells, its mortality, and (most of all) various behaviors and byproducts just south of the navel. We have lofty ideas, we fart, and we die. Not, one hopes, all at the same time, though the vulnerability is disobliging in any event.

McGinn runs through various theories about the dynamics and evolutionary benefits of disgust, and his phenomenological treatment of the feeling shares a lot with the thinking of other authors. Whatever the origins and cross-cultural variations, though, his real interest is in what it means that we are uniquely disgust-prone creatures. The salient thing is that disgust is the mind’s unhappy response to evidence that it is trapped in an animal that will one day rot. It is also an attempt to control that animal by putting a damper on human desire, which is polymorphous and insatiable: “Disgust is the human psyche policing itself, putting up self-imposed barriers.”

The problem being, of course, that the barriers don’t hold. A mind that polices itself can also plea-bargain with itself. The human condition seems defined by the relationship between desire and disgust, which seems unstable and treacherous. They can also blend together in all sorts of really strange ways. (It seems at times as if the Internet exists to document this fact.)

McGinn’s most influential work has been on the mind-body problem. “We know,” he wrote in a major paper, “that brains are the de facto causal basis of consciousness, but we have, it seems, no understanding whatever of how this can be so…. Neural transmissions just seem like the wrong kind of materials with which to bring consciousness into the world, but it appears that in some way they perform this mysterious feat.”

His argument is that we can’t bridge the gap between brain pulp and the mind’s eye, and never will. No need for dualism. We can assume that the mind is indeed a product of the brain’s operations. And we can learn more and more about those operations themselves. But the existence of the mind will remain mind-boggling, even so. Human thought is a puzzle for itself that, because of some limitation in its powers, it cannot solve.

One response to this is awe. Another, which comes more frequently, is disgust. McGinn’s understanding of disgust as a response to excess (a brake on unchecked desire) is a long way from treating it as a wellspring of reliable moral guidance, though. It is one more manifestation of the mind-body problem: the unpleasant reminder that a creature able to imagine its own immortality is "born between urine and feces," as St. Augustine was rude enough to put it.