The 100 Most Influential Scientists of All Time

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7 Introduction 7 In science the credit goes to the man who convinces the world, not to the man to whom the idea first occurs. —Francis Darwin (1848–1925) rom the very first moment humans appeared on the Fplanet, we have attempted to understand and explain the world around us. The most insatiably curious among us often have become scientists. The scientists discussed in this book have shaped humankind’s knowledge and laid the foundation for virtu- ally every scientific discipline, from basic biology to black holes. Some of these individuals were inclined to ponder questions about what was contained within the human body, while others were intrigued by celestial bodies. Their collective vision has been concentrated enough to exam- ine microscopic particles and broad enough to unlock tremendous universal marvels such as gravity, relativity— even the nature of life itself. Acknowledgement of their importance comes from a variety of knowledgeable and well-respected sources; luminaries such as Isaac Asimov and noted biochemist Marcel Florkin have written biogra- phies contained herein. The influence wielded by the profiled men and women within the realm of scientific discovery becomes readily apparent as the reader delves deeper into each individual’s life and contributions to his or her chosen field. Oftentimes, more than one field has been the beneficiary of these bril- liant minds. Many early scientists studied several different branches of science during their lifetimes. Indeed, as the founder of formal logic and the study of chemistry, biol- ogy, physics, zoology, botany, psychology, history, and literary theory in the Middle Ages, Aristotle is considered one of the greatest thinkers in history. Breakthroughs in the medical sciences have been numerous and extremely valuable. Study in this discipline 97 The 100 Most Influential Scientists of All Time 7 begins with a contemporary of Aristotle’s named Hippocrates, who is commonly regarded as the “father of medicine.” Perhaps Hippocrates’ most enduring legacy to the field is the Hippocratic Oath, the ethical code that doctors still abide by today. By taking the Hippocratic Oath, doctors pledge to Asclepius, the Greco-Roman god of medicine, that to the best of their knowledge and abili- ties, they will prescribe the best course of medical care for their patients. They also promise to, above all, cause no harm to any patient. The Greeks were not the only ones studying medi- cine. The Muslim scholar Avicenna also advanced the discipline by writing one of the most influential medical texts in history, The Canon of Medicine. Avicenna also pro- duced an encyclopedic volume describing Aristotle’s philosophic and scientific thoughts about logic, biology, psychology, geometry, astronomy, music, and metaphys- ics. This hefty tome was called the Kitāb al-shifā (“Book of Healing”). About 450 years later, a German-Swiss physi- cian named Philippus Aureolus Theophrastus Bombastus Von Hohenheim, or Paracelsus, once again advanced medical science by integrating medicine with chemistry and linking specific diseases to medications that could treat them. The Renaissance period brought to light the scientific genius of painter and sculptor Leonardo da Vinci. His drawings of presciently detailed flying machines preceded the advent of human flight by more than 300 years. What’s more, da Vinci’s drawings of the human anatomy struc- ture not only illuminated many of the body’s features and functions, they also laid the foundation for modern scien- tific illustration. Anatomical drawings were also the purview of Flemish physician Andreas Vesalius. Unlike da Vinci’s illustrations, 107 Introduction 7 which were mainly for his own artistic education, Vesalius incorporated his sketches and the explanations of them into the first anatomy textbook. His observations of human anatomy also helped to advance physiology, the study of the way the body functions. Other physicians took their investigation of anatomy off the page and onto the operating table. Ancient Greek physician Galen of Pergamum greatly influenced the study of medicine by performing countless autopsies on mon- keys, pigs, sheep, and goats. His observations allowed him to ascertain the functions of the nervous system and note the difference between arteries and veins. Galen was also able to dispel the notion that arteries carry air, an idea that had persisted for 400 years. Centuries later, in the 1600s, Englishman William Harvey built on Galen’s theories and observations, and helped lay the foundation for modern physiology with his numerous animal dissections. As a result of his work, Harvey was the first person to describe the function of the circulatory system, providing evidence that veins and arteries had separate and distinct functions. Before his realization that the heart acts as a pump that keeps blood flowing throughout the body, people thought that con- strictions of the blood vessels caused the blood to move. Other groundbreaking scientists have relied on obser- vations outside the body. A gifted Dutch scientist and lens grinder named Antonie van Leeuwenhoek refined the main tool of his trade, the microscope, which allowed him to become the first person to observe tiny microbes. Leeuwenhoek’s observations helped build the framework for bacteriology and protozoology. As several of the stories in this book confirm, science is a competitive yet oddly cooperative field, with research- ers frequently either refuting or capitalizing on one 117 The 100 Most Influential Scientists of All Time 7 another’s findings. Some ideas survive the test of time and remain intact while others are discarded or changed to fit more recent data. As an example of the former, Sir Isaac Newton developed three laws of motion that are still the basic tenets of mechanics to this day. Newton also proved instrumental to the advancement of science when he invented calculus, a branch of mathematics used by physi- cists and many others. Then there are the numerous advances made in the name of science that began with the development of vac- cines. Smallpox was a leading cause of death in 18th-century England. Yet Edward Jenner, an English surgeon, noticed something interesting occurring in his small village. People who were exposed to cowpox, a disease contracted from infected cattle that had relatively minor symptoms, did not get smallpox when they were exposed to the disease. Concluding that cowpox could protect people from small- pox, Jenner purposely infected a young boy who lived in the village first with cowpox, then with smallpox. Thankfully, Jenner’s hypothesis proved to be correct. He had successfully administered the world’s first vaccine and eradicated the disease. More than fifty years later, another scientist by the name of Louis Pasteur would expand Jenner’s ideas by explaining that the microbes, first discovered by Leeuwenhoek, caused diseases like smallpox. Today this idea is called the germ theory. Pasteur would go on to dis- cover the vaccines for anthrax, rabies, and other diseases. He also came to understand the role microbes played in the contamination and spoilage of food. The process he invented to prevent these problems, known as pasteuriza- tion, is still in use today. Other scientists, including Joseph Lister, Robert Koch, Sir Alexander Fleming, Selman Waksman, and 127 Introduction 7 Jonas Salk, would build on Pasteur’s germ theory, leading to subsequent discoveries of medical import. Anyone who ever needs to have an operation has Lister, the founder of antiseptic medicine, to thank for today’s ster- ile surgical techniques. Koch, with his numerous experiments and meticulous record keeping, was instru- mental in advancing the idea that particular microbes caused particular illnesses, greatly improving diagnostic medicine. Fleming was responsible for discovering the first antibiotic, penicillin, in 1928. Fleming’s work was continued by Waksman, who systematically searched for other antibiotics. This led to the discovery of one of the most widely used antibiotics of modern times, strepto- mycin, in 1943. Less than 10 years later, Salk would develop a vaccine that could protect children from the debilitating and deadly disease poliomyelitis. Since that time, scientists have almost succeeded in eliminating polio worldwide. Medical scientists are certainly not the only ones to build on one another’s work. Discoveries of one scientist, no matter what field he or she works in, are almost always examined, recreated, and expanded on by others. Luigi Galvani, an Italian physicist and physician, for example, discovered that animal tissue (specifically frog legs) could conduct an electric current. Building on Galvani’s obser- vations, his friend, Italian scientist Alessandro Volta, constructed the first battery in 1800. Expanding on Volta’s work and that of Danish physi- cist Hans Christiaan Ørsted, who discovered that electricity running through a wire could deflect a magnetic compass needle, French physicist André-Marie Ampère founded a new scientific field called electromagnetism. The English physicist Michael Faraday would pick up the work from there, using a magnetic field to produce an 137 The 100 Most Influential Scientists of All Time 7 electric current. In turn, this enabled him to invent and build the first electric motor. Reviewing Faraday’s experiments and theoretical work allowed James Clerk Maxwell to unify the ideas of elec- tricity and magnetism into an electromagnetic theory and to mathematically describe the electromagnetic force. Another physicist, Albert Michelson, determined that the speed of light was a never-changing constant. Using Maxwell’s mathematical theories and Michelson’s experi- mental data, Albert Einstein was able to develop his special theory of relativity, which resulted in what is arguably the 2 most famous equation in the world: E=mc . This elegantly simple but extremely powerful equation states that mass and energy are two different forms of the same thing. In other words, they are interchangeable. This idea has been indescribably important to the development of modern physics and astronomy. Einstein suggested that his idea could be tested using radium, a radioactive element discovered shortly before he announced his special theory of relativity. Discovered by Marie Curie, a Polish-born French chemist, and her husband, Pierre, radium continuously converts some of its mass into energy, a process Madame Curie named radioac- tivity. Her studies would eventually result in her becoming the first woman to ever be awarded a Nobel Prize. She was awarded a second Nobel Prize in 1911 for the discovery of polonium and radium. Building on the work of Curie and Einstein, future sci- entists would be successful—for better or worse—in harnessing nuclear energy. These concepts would be used to build fission reactors in nuclear power plants, produc- ing electricity for towns and cities. However, the same concepts would also be used by a group of scientists, including Enrico Fermi, J. Robert Oppenheimer, Luis Alvarez, and many others, to develop nuclear weapons. 147 Introduction 7 In 1675, Isaac Newton wrote a letter to Robert Hooke in which he said, “If I have seen further it is by standing on the shoulders of giants.” Thanks to the pioneering efforts of the scientists mentioned in this introduction, along with the other chemists, biologists, astronomers, ecologists, and geneticists in the remainder of this book, today’s scientists have a solid foundation upon which to make astounding leaps of logic. Without the work of these men and women, we would not have computers, electricity, or many other modern conveniences. We would not have the vaccines and medications that help keep us healthy. And, in general, we would know a lot less about the way the human body functions and the way the world works. Today’s scientists owe a huge debt of gratitude to the scientists of days past. By standing on the shoulders of these giants, who knows how far they may be able to see. 157 Asclepius 7 ASCLEPIUS n the Iliad, the writer Homer mentions Asclepius only as Ia skillful physician and the father of two Greek doctors at Troy, Machaon and Podalirius. In later times, however, he was honoured as a hero, and eventually worshiped as a god. Asclepius (Greek: Asklepios, Latin: Aesculapius), the son of Apollo (god of healing, truth, and prophecy) and the mortal princess Coronis, became the Greco-Roman god of medicine. Legend has it that the Centaur Chiron, who was famous for his wisdom and knowledge of medi- cine, taught Asclepius the art of healing. At length Zeus, the king of the gods, afraid that Asclepius might render all men immortal, slew him with a thunderbolt. Apollo slew the Cyclopes who had made the thunderbolt and was then forced by Zeus to serve Admetus. Asclepius’s cult began in Thessaly but spread to many parts of Greece. Because it was supposed that Asclepius effected cures of the sick in dreams, the practice of sleep- ing in his temples in Epidaurus in South Greece became common. This practice is often described as Asclepian incubation. In 293 BCE his cult spread to Rome, where he was worshiped as Aesculapius. Asclepius was frequently represented standing, dressed in a long cloak, with bare breast; his usual attribute was a staff with a serpent coiled around it. This staff is the only true symbol of medicine. A similar but unrelated emblem, the caduceus, with its winged staff and intertwined ser- pents, is frequently used as a medical emblem but is without medical relevance since it represents the magic wand of Hermes, or Mercury, the messenger of the gods and the patron of trade. However, its similarity to the staff of Asclepius resulted in modern times in the adoption of the caduceus as a symbol of the physician and as the emblem of the U.S. Army Medical Corp. 177 The 100 Most Influential Scientists of All Time 7 The plant genus Asclepias, which contains various species of milkweed, was named for Asclepius. Many of these plants possess some degree of medicinal value. HIPPoCrA tES (b. c. 460 BCE, island of Cos, Greece—d. c. 375 BCE, Larissa, Thessaly) ippocrates was an ancient Greek physician who lived Hduring Greece’s Classical period and is traditionally regarded as the father of medicine. It is difficult to isolate the facts of Hippocrates’ life from the later tales told about him or to assess his medicine accurately in the face of cen- turies of reverence for him as the ideal physician. About 60 medical writings have survived that bear his name, most of which were not written by him. He has been revered for his ethical standards in medical practice, mainly for the Hippocratic Oath, which, it is suspected, he did not write. Life and Works What is known is that while Hippocrates was alive, he was admired as a physician and teacher. In the Protagoras Plato called Hippocrates “the Asclepiad of Cos,” who taught students for fees. Further, he implied that Hippocrates was as well known as a physician as Polyclitus and Phidias were as sculptors. Plato also referenced Hippocrates in the Phaedrus, in which Hippocrates is referred to as a famous Asclepiad who had a philosophical approach to medicine. Meno, a pupil of Aristotle, specifically stated in his his - tory of medicine the views of Hippocrates on the causation of diseases, namely, that undigested residues were produced by unsuitable diet and that these residues excreted vapours, which passed into the body generally and produced 187 Hippocrates 7 diseases. Aristotle said that Hippocrates was called “the Great Physician” but that he was small in stature. Hippocrates appears to have traveled widely in Greece and Asia Minor practicing his art and teaching his pupils. He presumably taught at the medical school at Cos quite frequently. His reputation, and myths about his life and his family, began to grow in the Hellenistic period, about a century after his death. During this period, the Museum of Alexandria in Egypt collected for its library literary material from preceding periods in celebration of the past greatness of Greece. So far as it can be inferred, the medi- cal works that remained from the Classical period (among the earliest prose writings in Greek) were assembled as a group and called the works of Hippocrates (Corpus Hippocraticum). The virtues of the Hippocratic writings are many, and, although they are of varying lengths and literary quality, they are all simple and direct, earnest in their desire to help, and lacking in technical jargon and elaborate argu- ment. The works show such different views and styles that they cannot be by one person, and some were clearly writ- ten in later periods. Yet all the works of the Corpus share basic assumptions about how the body works and what disease is, providing a sense of the substance and appeal of ancient Greek medicine as practiced by Hippocrates and other physicians of his era. Prominent among these attrac- tive works are the Epidemics, which give annual records of weather and associated diseases, along with individual case histories and records of treatment, collected from cities in northern Greece. Diagnosis and prognosis are frequent subjects. Other treatises explain how to set fractures and treat wounds, feed and comfort patients, and take care of the body to avoid illness. Treatises called Diseases deal with serious illnesses, proceeding from the head to the feet, 197 The 100 Most Influential Scientists of All Time 7 giving symptoms, prognoses, and treatments. There are works on diseases of women, childbirth, and pediatrics. Prescribed medications, other than foods and local salves, are generally purgatives to rid the body of the noxious sub- stances thought to cause disease. Some works argue that medicine is indeed a science, with firm principles and methods, although explicit medical theory is very rare. The medicine depends on a mythology of how the body works and how its inner organs are connected. The myth is laboriously constructed from experience, but it must be remembered that there was neither systematic research nor dissection of human beings in Hippocrates’ time. Hence, while much of the writing seems wise and correct, there are large areas where much is unknown. Over the next four centuries, imaginative writings, some obviously fiction, were added to the original collec- tion of Hippocratic works and enhanced Hippocrates’ reputation, providing the basis for the traditional picture of Hippocrates as the father of medicine. Still other works were added to the Hippocratic Corpus between its first collection and its first scholarly edition around the begin- ning of the 2nd century CE. Among them were the Hippocratic Oath and other ethical writings that pre- scribe principles of behaviour for the physician. Hippocratic Oath The Hippocratic Oath dictates the obligations of the physician to students of medicine and the duties of pupil to teacher. In the oath, the physician pledges to prescribe only beneficial treatments, according to his abilities and judgment; to refrain from causing harm or hurt; and to live an exemplary personal and professional life. The text of the Hippocratic Oath (c. 400 BCE) provided below is a translation from Greek by Francis Adams (1849). It is 207 Hippocrates 7 considered a classical version and differs from contempo- rary versions, which are reviewed and revised frequently to fit with changes in modern medical practice. I swear by Apollo the physician, and Aesculapius, and Health, and All-heal, and all the gods and goddesses, that, according to my ability and judgment, I will keep this Oath and this stipulation—to reckon him who taught me this Art equally dear to me as my parents, to share my substance with him, and relieve his necessities if required; to look upon his offspring in the same footing as my own brothers, and to teach them this Art, if they shall wish to learn it, without fee or stipulation; and that by precept, lecture, and every other mode of instruction, I will impart a knowledge of the Art to my own sons, and those of my teachers, and to disciples bound by a stipulation and oath according to the law of medicine, but to none others. I will follow that system of regimen which, according to my ability and judgment, I consider for the benefit of my patients, and abstain from whatever is del- eterious and mischievous. I will give no deadly medicine to any one if asked, nor suggest any such counsel; and in like manner I will not give to a woman a pessary to produce abortion. With purity and with holiness I will pass my life and practice my Art. I will not cut persons laboring under the stone, but will leave this to be done by men who are prac- titioners of this work. Into whatever houses I enter, I will go into them for the benefit of the sick, and will abstain from every voluntary act of mischief and corruption; and, further from the seduction of females or males, of freemen and slaves. Whatever, in connection with my professional practice or not, in connection with it, I see or hear, in the life of men, which ought not to be spoken of abroad, I will not divulge, as reckoning that all such should be kept secret. While I con- tinue to keep this Oath unviolated, may it be granted to me to enjoy life and the practice of the art, respected by all men, 217 The 100 Most Influential Scientists of All Time 7 in all times But should I trespass and violate this Oath, may the reverse be my lot Influence Technical medical science developed in the Hellenistic period and after. Surgery, pharmacy, and anatomy advanced; physiology became the subject of serious spec- ulation; and philosophic criticism improved the logic of medical theories. Competing schools in medicine (first Empiricism and later Rationalism) claimed Hippocrates as the origin and inspiration of their doctrines. For later physicians, Hippocrates stood as the inspirational source, and today Hippocrates still continues to represent the humane, ethical aspects of the medical profession. ArISLE tot (b. 384 BCE, Stagira, Chalcidice, Greece—d. 322 BCE, Chalcis, Euboea) ristotle (Greek: Aristoteles) was an ancient Greek Aphilosopher and scientist, and one of the greatest intellectual figures of Western history. He was the author of a philosophical and scientific system that became the framework and vehicle for both Christian Scholasticism and medieval Islamic philosophy. Aristotle’s intellectual range was vast, covering most of the sciences and many of the arts, including biology, botany, chemistry, ethics, his- tory, logic, metaphysics, rhetoric, philosophy of mind, philosophy of science, physics, poetics, political theory, psychology, and zoology. He was the founder of formal logic, devising for it a finished system that for centuries was regarded as the sum of the discipline. Aristotle also pioneered the study of zoology, both observational and theoretical, in which some of his work remained unsur- passed until the 19th century. His writings in metaphysics 227 Aristotle 7 This statue of Aristotle, the Greek philosopher who taught Alexander the Great, stands in the Palazzo Spada in Rome. Popperfoto/Getty Images 237 The 100 Most Influential Scientists of All Time 7 and the philosophy of science continue to be studied, and his work remains a powerful current in contemporary philosophical debate. Physics and Metaphysics Aristotle divided the theoretical sciences into three groups: physics, mathematics, and theology. Physics as he understood it was equivalent to what would now be called “natural philosophy,” or the study of nature; in this sense it encompasses not only the modern field of physics but also biology, chemistry, geology, psychology, and even meteo- rology. Metaphysics, however, is notably absent from Aristotle’s classification; indeed, he never uses the word, which first appears in the posthumous catalog of his writ- ings as a name for the works listed after the Physics. He does, however, recognize the branch of philosophy now called metaphysics. He calls it “first philosophy” and defines it as the discipline that studies “being as being.” Aristotle’s contributions to the physical sciences are less impressive than his researches in the life sciences. In works such as On Generation and Corruption and On the Heavens, he presented a world-picture that included many features inherited from his pre-Socratic predecessors. From Empedocles (c. 490–430 BCE) he adopted the view that the universe is ultimately composed of different com- binations of the four fundamental elements of earth, water, air, and fire. Each element is characterized by the possession of a unique pair of the four elementary quali- ties of heat, cold, wetness, and dryness: earth is cold and dry, water is cold and wet, air is hot and wet, and fire is hot and dry. Each element also has a natural place in an ordered cosmos, and each has an innate tendency to move toward this natural place. Thus, earthy solids naturally fall, while 247 Aristotle 7 fire, unless prevented, rises ever higher. Other motions of the elements are possible but are considered “violent.” (A relic of Aristotle’s distinction is preserved in the modern- day contrast between natural and violent death.) Aristotle’s vision of the cosmos also owes much to Plato’s dialogue Timaeus. As in that work, the Earth is at the centre of the universe, and around it the Moon, the Sun, and the other planets revolve in a succession of concentric crystalline spheres. The heavenly bodies are not com- pounds of the four terrestrial elements but are made up of a superior fifth element, or “quintessence.” In addition, the heavenly bodies have souls, or supernatural intellects, which guide them in their travels through the cosmos. Even the best of Aristotle’s scientific work has now only a historical interest. The abiding value of treatises such as the Physics lies not in their particular scientific assertions but in their philosophical analyses of some of the concepts that pervade the physics of different eras— concepts such as place, time, causation, and determinism. Philosophy of Science In his Posterior Analytics, Aristotle applies the theory of the syllogism (a form of deductive reasoning) to scientic fi and epistemological ends (epistemology is the philosophy of the nature of knowledge). Scientic fi knowledge, he urges, must be built up out of demonstrations. A demonstration is a particular kind of syllogism, one whose premises can be traced back to principles that are true, necessary, uni- versal, and immediately intuited. These r fi st, self-evident principles are related to the conclusions of science as axi- oms are related to theorems: the axioms both necessitate and explain the truths that constitute a science. The most important axioms, Aristotle thought, would be those that 257 The 100 Most Influential Scientists of All Time 7 define the proper subject matter of a science. Thus, among the axioms of geometry would be the definition of a tri- angle. For this reason much of the second book of the Posterior Analytics is devoted to definition. The account of science in the Posterior Analytics is impressive, but it bears no resemblance to any of Aristotle’s own scientific works. Generations of scholars have tried in vain to find in his writings a single instance of a demon- strative syllogism. Moreover, the whole history of scientific endeavour contains no perfect instance of a demonstra- tive science. PLI ny tHE ELE d r (b. 23 CE, Novum Comum, Transpadane Gaul now in Italy—d. Aug. 24, 79, Stabiae, near Mt. Vesuvius) liny the Elder (Latin: Gaius Plinius Secundus) was a PRoman savant and author of the celebrated Natural History, an encyclopaedic work of uneven accuracy that was an authority on scientific matters up to the Middle Ages. Seven writings are ascribed to Pliny, of which only the Natural History is extant. There survive, however, a few fragments of his earlier writings on grammar, a biography of Pomponius Secundus, a history of Rome, a study of the Roman campaigns in Germany, and a book on hurling the lance. These writings probably were lost in antiquity and have played no role in perpetuating Pliny’s fame, which rests solely on the Natural History. The Natural History, divided into 37 libri, or “books,” was completed, except for finishing touches, in 77 CE. In the preface, dedicated to Titus (who became emperor shortly before Pliny’s death), Pliny justified the title and explained his purpose on utilitarian grounds as the study of “the nature of things, that is, life.” Heretofore, he con- tinued, no one had attempted to bring together the older, 267 Pliny the Elder 7 scattered material that belonged to “encyclic culture” (enkyklios paideia, the origin of the word encyclopaedia). Disdaining high literary style and political mythology, Pliny adopted a plain style—but one with an unusually rich vocabulary—as best suited to his purpose. A novel feature of the Natural History is the care taken by Pliny in naming his sources, more than 100 of which are men- tioned. Book I, in fact, is a summary of the remaining 36 books, listing the authors and sometimes the titles of the books (many of which are now lost) from which Pliny derived his material. The Natural History properly begins with Book II, which is devoted to cosmology and astronomy. Here, as elsewhere, Pliny demonstrated the extent of his reading, especially of Greek texts. By the same token, however, he was sometimes careless in translating details, with the result that he distorted the meaning of many technical and mathematical passages. In Books III through VI, on the physical and historical geography of the ancient world, he gave much attention to major cities, some of which no longer exist. Books VII through XI treat zoology, beginning with humans, then mammals and reptiles, fishes and other marine animals, birds, and insects. Pliny derived most of the biological data from Aristotle, while his own contribu- tions were concerned with legendary animals and unsupported folklore. In Books XII through XIX, on botany, Pliny came closest to making a genuine contribution to science. Although he drew heavily upon Theophrastus, he reported some independent observations, particularly those made during his travels in Germany. Pliny is one of the chief sources of modern knowledge of Roman gardens, early botanical writings, and the introduction into Italy of new horticultural and agricultural species. Book XVIII, on 277 The 100 Most Influential Scientists of All Time 7 agriculture, is especially important for agricultural tech- niques such as crop rotation, farm management, and the names of legumes and other crop plants. His description of an ox-driven grain harvester in Gaul, long regarded by scholars as imaginary, was confirmed by the discovery in southern Belgium in 1958 of a 2nd-century stone relief depicting such an implement. Moreover, by recording the Latin synonyms of Greek plant names, he made most of the plants mentioned in earlier Greek writings identifiable. Books XX through XXXII focus on medicine and drugs. Like many Romans, Pliny criticized luxury on moral and medical grounds. His random comments on diet and on the commercial sources and prices of the ingredients of costly drugs provide valuable evidence relevant to contem- porary Roman life. The subjects of Books XXXIII through XXXVII include minerals, precious stones, and metals, especially those used by Roman craftsmen. In describing their uses, he referred to famous artists and their creations and to Roman architectural styles and technology. Influence Perhaps the most important of the pseudoscientific methods advocated by Pliny was the doctrine of signa- tures: a resemblance between the external appearance of a plant, animal, or mineral and the outward symptoms of a disease was thought to indicate the therapeutic useful- ness of the plant. With the decline of the ancient world and the loss of the Greek texts on which Pliny had so heavily depended, the Natural History became a substi- tute for a general education. In the European Middle Ages many of the larger monastic libraries possessed cop- ies of the work. These and many abridged versions ensured Pliny’s place in European literature. His authority was 28