The eighteenth century has been aptly described as the adolescence of modern medicine, the era in which the foundations of scientiﬁc medicine were ﬁrst established. During this period, the ideas of the philosophical movement known as the Enlightenment inspired the search for rational systems of medicine, practical means of preventing disease, improving the human condition, and disseminating the new learning to the greatest number of people possible. Although historians continue to argue about the deﬁnition, meaning, and even the existence of a speciﬁc era popu- larly known as the Enlightenment, there is general agreement that Enlightenment thought may be interpreted as the optimistic belief that human beings have the capacity to subject all inherited beliefs to rational analysis and open debate. The eighteenth century boasts a prodigious who’s who of physicians and scientists, easy to list, but impos- sible to discuss in the detail they deserve. A few of the leading lights of clinical medicine will have to serve as exemplars of this era. Just as Thomas Sydenham is honored for following Hippocrates in his emphasis on patient care and epidemiological observations, Hermann Boerhaave (1668–1738) is remembered for his role in revitalizing the teaching of clinical medicine. Teacher, writer, and chemist, Boerhaave was probably the most inﬂuential physician of the eighteenth century. His contemporaries thought of him as the ‘‘Newton of Medicine.’’ Speaking of his own sources of inspiration, Boerhaave emphasized the work of Hippocrates, Francis Bacon, and Thomas Sydenham. It was said that, in deference to the ‘‘English Hippocrates,’’ Boerhaave tipped his hat every time Sydenham’s name was mentioned. As a student, Boerhaave immersed himself in botany, chemistry, philosophy, and languages. Although, like Sydenham, he suffered the torments of gout, Boerhaave possessed boundless energy as well as erudition, as demonstrated by his simultaneous commitment to professorships in botany, chemistry, medical theory, and clinical medicine at Leiden. By establishing a hospital especially for teaching purposes, Boerhaave was able to combine theoretical and practical instruction at the patient’s bedside. Bedside instruction, which remedied one of the greatest deﬁciencies of academic medicine, made Leiden a major center of medical education—at least until Boerhaave’s disciples succeeded in bringing clinical instruction to other schools. Bedside medicine prepared the way for hospital medicine, which developed during the last years of the eighteenth century and ﬂourished in the nineteenth century. No major biomedical discovery can be attributed to Boerhaave, but medical students were taught to think of his system as ‘‘perfect, com- plete, and sufﬁcient,’’ and powerful enough to ﬁll the void created by the demise of Galenism. Those who dared to differ from the great Boerhaave were denounced as medical heretics. Through lectures faith- fully recorded by his disciples, Boerhaave became teacher to the world. The books that expressed Boerhaave’s ideas, Institutiones medicae (1708), Book of Aphorisms (1709), Index plantarum (1710), and Elementia chemiae (1732), remained in use for almost one hundred years. Boerhaave taught his followers that the study of human health and disease must be based on anatomy and physiology, chemistry and physics. Unfor- tunately, like Galenism, Boerhaave’s beautifully crafted system so thoroughly satisﬁed his contemporaries that it tended to stiﬂe curiosity and innovation. The great virtue, and ultimately the equally great ﬂaw, of Boerhaave’s system was the way he integrated classiﬁcation and natural science with considerations of the nature, causes, and treatment of dis- ease. For example, eighteenth-century physiologists regarded the study of the chemistry of digestion as part of a deep philosophical argument about life. If one could understand how the dead matter of plant and animal foods could nourish living tissue, one would discover the secret of life. According to Boerhaave, if the digestive processes were inade- quate, various foods gave rise to acids. Since acids were supposedly foreign to bodily humors, the resultant state of acid acrimony would produce disorders of the intestinal tract, which then affected the blood, milk, skin, and brain. Obviously, such disorders should be treated with anti-acids such as meat, ﬁsh, leafy vegetables, and alkaline powders. The chemistry of Boerhaave’s medical system is intriguing, but often confusing, because terms that seem familiar to modern chemists meant something quite different within an eighteenth century context. For example, earth signiﬁed an inert material that could not be liqueﬁed by ﬁre or dissolved in water. A salt was a substance that dissolved in water and was liqueﬁed by ﬁre. Sulfur and oil were substances that melted and burned, but did not mix with water. Eventually, the deﬁciencies and failures of eighteenth-century medical systems became all too apparent. New discover- ies and unlovely facts forced nineteenth-century physicians to conﬁne them- selves to formulating more modest and limited explanatory frameworks. The eighteenth century is also notable for the work of Giovanni Battista Morgagni (1682–1771), author of De sedibus et causis morborum (On the Seat and Cause of Disease, 1761) a ﬁve-volume landmark in the evolution of pathological anatomy. After studying medicine in Bologna, Morgagni became Professor of Theoretical Medicine and Anatomy at the University of Padua. Like his predecessor, Andreas Vesalius, Morgagni brought great glory to the University through his anatomical research. Morgagni’s attempt to ﬁnd correlations between clinical symptoms and postmortem ﬁndings was based on over six hundred dis- sections. Careful observations of the appearance and course of various diseases were essential to Morgagni’s research program, as were dis- sections and experiments on various animals as a means of understanding clinical patterns of disease in humans. Convinced that normal human anatomy had been well established, Morgagni focused his considerable energies on exploring the origin and seat of diseases that caused pathological changes demonstrable in the cadaver. His meticulous observations and case histories were carefully arranged and published when he was 80 years old. After summarizing each case history, Morgagni attempted to correlate observations of the course of illness with his ﬁndings at autopsy. Autopsies sometimes revealed errors in diagnosis and treatment that contributed to, or caused, the death of the patient. In one case, the attending physician had diag- nosed a stomach complaint, but the postmortem revealed that the patient had a normal stomach and diseased kidneys. Autopsies sometimes revealed sad and bizarre behaviors. For example, in discussing ﬁndings related to the suppression of urine, Morgagni noted several cases where girls had damaged the bladder by introducing needles or pins into the urethra. Some girls claimed to have swallowed these items, but others tried to conceal their injuries, even if it meant death. Similarly, at autopsy Morgagni discovered that some males had died with needles in their urinary organs. Morgagni boasted that he had dissected more male urethras than any other anat- omist, but he complained that he had not found as many cases of damage to the urethra due to gonorrhea as he had expected. He was unsuccessful in discovering the seat of gonorrhea in males and females, but dissections did prove that over the course of many years the disease insidiously made its way throughout the body. In compiling case studies, the morbid anatomist needed a healthy dose of skepticism. Colleagues offered Morgagni reports of bizarre hun- gers brought on by lice growing in the stomach and worms in the appen- dix. He was quite suspicious of the ﬁrst report, but thought that the second seemed plausible. In discussing various kinds of ﬂuxes (diar- rheas) he urged his readers to be suspicious when evaluating reports of the ingestion or excretion of frogs, toads, lizards, and so forth. The anatomist should examine the physical evidence and determine what kind of bodily parts and products were actually involved. Morgagni is regarded as a pioneer of morbid anatomy and a guide to a new epoch in medical science. Even though Morgagni remained essentially a humoralist, his work may be seen as part of the transition from general humoral pathology towards the study of localized lesions and diseased organs. By encouraging physicians to think of disease in terms of localized pathological changes rather than disorders of the humors, Morgagni’s work brought about a new attitude towards spe- ciﬁc diagnostic and surgical interventions. He was the ﬁrst person to attempt a systematic examination of the connection between the symp- toms of disease in the living body and post-mortem results revealed only to the dedicated investigator. His work helped to establish an anatom- ical orientation in pathology and the recognition that unseen anatom- ical changes within the body were reﬂected in the clinical picture. Conﬁrmation could be found only in the autopsy room, but recognition of the relationship encouraged interest in ﬁnding ways of anatomiz- ing the living—that is, detecting hidden anatomical lesions in living patients. This goal would be realized in Leopold Auenbrugger’s (1722–1809) studies of chest percussion, Rene´ The´ophile Hyacinthe Lae¨nnec’s (1781–1826) invention of the stethoscope, and the remarkable rise of medical instrumentation that followed.