Pain is the Sasquatch of science, never witnessed, only endlessly speculated on. We can't even agree on the species. Man or beast? A sensation or an idea? It doesn't help that ideas about the meanings of pain are double-barreled abstractions that soon drift away from the experience itself into an epistemological fog.

Our efforts to describe pain soon confront us with another small problem: How do we define the self? What particular nexus of mind, body, and soul is this modern "I" who feels the strange brew of modern pain?

I've been ruthlessly selective in this chapter, skipping over many names and entire centuries, to avoid disappearing down philosophical cul-de-sacs. But as I began to investigate the earliest ideas of pain, what struck me was that philosophy, medicine, and drama were once much closer in the way they viewed pain. It wasn't until Descartes came along in the seventeenth century with "proof" of the mind-body split, followed by the age of Enlightenment, that pain began to shed its emotional and social dimensions. One of the earliest definitions of tragedy, for instance, was human pain-as our exile into something that can be witnessed and pitied, but never shared.

Philoctetes, a play written by Sophocles in 409 b.c., is a story that pivots around the physical pain of its main character, who suffers from a wound that began as a snake bite. "Terrible it is, beyond words' reach" is how Philoctetes describes his condition. This inviolate, unspeakable aspect of human pain is what the drama tries to voice. "Philoctetes makes us feel the power of pain to reduce a life to utter emptiness and misery," author David Morris writes in The Culture of Pain. "It unweaves the self until the self is nothing but pain. The body in tragedy is not just something we possess like an identifying birthmark or robe or kingdom," Morris argues, "but what we are. It both defines us, and, fatally, limits us."

Aristotle was another astute observer of human dramas, including pain, and his writings on the subject turn out to have a rather modern flair. He defined pain as an emotion rather than a mechanical sensation. He characterized both pain and pleasure as "appetites" that drive us toward the objects of our desires and away from the things that hurt us. For Aristotle, pain was not only a sensory event in the body, but a subjective state, like longing and fear. He saw the human cost of pain, how it "upsets and destroys the nature of the person who feels it." Aristotle may not have understood physiology, but he accepted the idea that pain is an expression of who we are.

Our uncertainty about the province of pain is conveyed by the roots of the words we use for it. Pain is probably derived from the Latin word poena , meaning punishment, and the English word tends to connote physical pain. But the French word douleur , from the Latin dolor, refers to both physical and mental pain. The French word peine suggests punishment, but it can mean sorrow as well. Oddly enough, the Italian language has no word for ache, despite the fact that studies of pain expression in different cultures report that Italian women in labor are louder than women from other countries.

The concept of pain as punishment turns up most vividly in the biblical story of Job, a wealthy, upright man whose faith in God is tested by Satan in a series of terrible afflictions. First he loses his wealth, then he becomes an outcast from his community. Finally Satan pulls out all the stops and inflicts a "plague of boils" on Job. "He slashes open my kidneys and does not spare," says Job, describing Satan's work. "He pours out my gall on the ground." William Blake's illustration of this scene shows the figure of Job writhing on the ground, his hands arched back in pain, as a naked, burly Satan stands over him like a TV wrestler in triumph. Job's test of faith is the first example of the theme of bloody martyrdom that runs throughout Christianity. Pain is inseparable from faith and "the central Christian mystery of a being who suffers pain in order to redeem others," as Morris writes. It was the pain that Jesus Christ suffered on the cross that proved to us that God's son was human, too. Suffering pain is how faith is forged; transcending pain is a mark of sainthood. The image of St. Sebastian pierced with arrows, with upturned eyes, carries the message that a belief in a life beyond the body has the power to undo pain. The idea of pain as spiritual punishment is still deeply entrenched in our attitude that physical pain arrives as a kind of moral test of character and should be toughed out. The price of admission for being human, the story of Job reminds us, is this: expect boils.

Fast-forward to the Middle Ages, a time when it was hell to have a toothache, even though laudanum laced with opium was readily dispensed. One of the opiophiles of the era was the enlightened sixteenth-century practitioner Paracelsus. He was the original patient-centered physician. "Every physician must be rich in knowledge," he wrote in Man and His Body, "and not only of that which is written in books; his patients should be his book, they will never mislead him...and by them he will never be deceived. But he who is content with mere letters is like a dead man; and he is like a dead physician." We may be overdue for a Paracelsus revival.

The man most responsible for our modern misconception of "mental pain" versus "physical pain," however, was the seventeenth-century philosopher and scientist Rene Descartes. Although he is often blamed for the mind-body split that came to characterize Western thinking, in other ways, Descartes's investigation into pain was farsighted. In the treatise De l'homme, his hypothesis about pain pathways and the "delicate threads" that conduct pain signals, for instance, turned out to be a crude but correct notion of nerve fibers and neurotransmitters. But it was his theory of the transmission of pain signals that led to what is known as the "specificity theory" of pain-the notion of pain as one fixed pathway or center. This idea dominated the study of pain until the last thirty or forty years.

Descartes's theory was accompanied by a famous illustration of a rather hunchbacked naked man, eyes a-bulge, who appears to be stepping into a campfire. His foot is in the flame. "If for example fire comes near the foot," he wrote in 1640, "minute particles of this fire, which you know move at great velocity, have the power to set in motion the spot of skin on the foot which they touch, and by this means pulling on the delicate thread which is attached to the spot of the skin, they open up at the same instant the pore against which the delicate thread ends, just as by pulling on one end of a rope one makes to strike at the same instant a bell which hangs at the end."

Descartes has helpfully labeled the diagram. The sensation of pain (A) is perceived in the foot and then travels up to the "common sense center" (F) in the pineal gland, which interprets the signal as pain. This same stimulus-response model still defines our popular understanding of pain: The coffee table hits your toe, a sensation in the nerves then tugs at the bell-rope of the brain, which interprets this event as pain. No coffee table, no pain. But even in his time, Descartes had to defend this theory against critics. When it was pointed out to him that some amputees still feel pain in their missing limbs-phantom limb pain-he nimbly responded that the brain was just being tricked by false signals. But he still characterized the mind as a passive central switchboard instead of as a coauthor of pain.

In Descartes's mechanistic view, pain is something that happens to the body, a sensation then promoted to the status of a concept in the brain. A worker-CEO arrangement, you could say, except that the goods flow only one way. Although the brain is the boss, it is a passive decoder, and pain only runs along one track, with its own special apparatus, impervious to emotions or environmental factors.

The race for pain's Northwest Passage-the path it takes in the body-was under way, and for the next three hundred years science pursued this mysterious trail. Pain began to lose its multiple meanings, as a visionary experience in religion, or as an expressive element of tragedy. Instead, pain became the property of science and medicine, even though they didn't quite know what to make of it. The focus shifted from exploring the questions of identity, consciousness, and grace that pain raises to describing its mechanisms in the body and brain. The pharmaceutical age began at the end of the nineteenth century. Cutting pain out of the body, cutting nerves, and killing pain became the new goals.

A time line of some of the landmarks of pain science and treatment over the past two centuries might look like this:

1803 Morphine is synthesized from opium

1846 The discovery of anesthesia

1853 The invention of the hypodermic needle

1853 Acetylsalicylic acid, predecessor to aspirin is developed

1914 The Harrison Act in the United States sets restrictions on narcotic drugs

1943 Pain Mechanisms published by William Livingston

1946 Henry Beecher's work on the power of the placebo

1965 The gate-control theory of pain published by Melzack and Wall in the journal Science

1966 The first hospice, St. Christopher's, opens in the United Kingdom

1973 International Association for the Study of Pain holds its first congress

1975 The McGill Pain Questionnaire (first measurement of pain intensity)

1976 Discovery of endorphins

1986 The World Health Organization publishes The Analgesic Ladder: Guidelines to Cancer Pain Relief

2000 The U.S. Congress declares the next ten years the "Decade of Pain Control and Research"

The distinction between "mental pain" and "physical pain," our legacy from Descartes, has led to a punishing skepticism about "real" pain versus "invented" pain. The specificity theory describes pain as an event in the periphery of the body that is open to interpretation, and distortion, by the mind; pain that couldn't be connected to an injury or some sort of organic cause was "psychological" and therefore suspect. This theory doesn't account for why one person can be more sensitive to pain than another, and it led to the belief that the intensity of pain is always in direct proportion to the intensity of the stimulus. But the lightest breath of air on the skin can cause severe pain for someone suffering the neuropathic pain known as reflex sympathetic dystrophy (RSD). Long after recovery from an injury, people with RSD can suffer chronic pain, to the exasperation of their doctors. The specificity theory made it possible to blame people for their own pain. Descartes could be called the father of malingering.

We now know that even the pain of a minor accident can sensitize the central nervous system in some people, as if the "on" switch for pain works, but the "off" switch is broken. People with phantom limb pain can suffer vivid, detailed pain in a hand or leg that no longer exists. Descartes was right about the fact that nerves in the periphery of the body carry signals to the brain, but not in the one-way uphill street he imagined. What science has discovered since then is that the spinal cord and central nervous system play major roles in pain perception. Descending messages from the brain can block or modify the sensory information coming in. As pain researchers Ron Melzack and Patrick Wall would demonstrate, pain is the result of a complex feedback loop.

Above all, pain is in the brain-not Descartes's passive, traffic-cop model, but one in which pain lights up multiple areas at once, a fluid, dynamic event responding to information from the senses at the same time that it shapes that response. Pain really is all in the head. But the brain doesn't just react to the foot in the flame. The body and the mind create a neural narrative together.

As neuroscience maps the brain in more detail, the gap between mind and body begins to narrow and to show itself for what it is-a false construct. The body begins to look much smarter and more soulful (flesh as "spirit thickened" as surgeon and author Richard Selzer has written) at the same time that the mind incarnates itself, as a biochemical event. Descartes's "bell" now includes not just skin, nerves, and sensation, but also memory, thoughts, and feelings.

In the nineteenth century, Silas Weir Mitchell, the father of neurology, collected case studies of nerve injuries in soldiers with gunshot wounds. His description of the mysterious burning pain of "causalgia" was part of the gradual shift away from this equation of injury to pain. What Mitchell described was a very real agony that had no obvious connection to tissue damage at all. Mitchell (to whom I will return later) also had some curious notions about women, hysteria, and pain, but he was an outstanding example of a departed nineteenth-century figure-a doctor who published both fiction and poetry, who worked both in the Weld and in the lab. Mitchell understood the relationship between pain and personal history and environment fifty years before the rest of science.

In the middle of the nineteenth century, the invention of anesthesia brought a measure of control over pain and enabled surgeons to do more complicated, lifesaving operations. We began to live longer as a result. Before anesthesia, surgery was a horrific cut-and-grab procedure that was performed as fast as possible by barbers. Anesthesiologists have been in the forefront of pain studies ever since. (The International Association for the Study of Pain, an organization of professionals in the pain Weld, was founded in 1973 by an anesthetist, John Bonica.) But the arrival of anesthesia also put the focus on erasing pain rather than exploring its role in health and disease. Anesthesiology doesn't target pain; it puts the patient in a twilight state-a kind of mock death, actually, with machines taking over the patient's vital functions. This demonstrates one of the most obvious qualities of pain: It requires a consciousness to feel it. What the unconscious patient feels as the knife cuts into him is unknown, but it's not what we call pain.

The idea of being "put to sleep" was greeted with some suspicion at first. Despite the fact that Queen Victoria gave birth to two of her children under the painkilling influence of ether, there was a cadre of obstetricians who violently opposed its use in labor. Pain was considered a necessary and natural part of giving birth, not to mention part of Eve's punishment for disobeying God.

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Excerpted from Pain: The Fifth Vital Sign by Marni Jackson. Copyright 2002 by Marni Jackson. Excerpted by permission of Random House of Canada, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.