In the 5th century BCE the Greek physician Hippocrates (?460–?377 BCE) clearly established the idea that disease was the result of natural forces rather than divine intervention, and that patients’ own powers of healing should be encouraged (see page 19). Contemporary medical theories were based upon the Law of Contraries, which advocated treating an illness by prescribing a substance that produced opposite or contrary symptoms. Diarrhea, for example, could be treated by a substance that caused constipation, such as aluminum hydroxide. In contrast, Hippocrates developed the use of the Law of Similars, based on the principle that “like cures like” (see page 18). This theory proposed that substances capable of causing symptoms of illness in healthy people could also be used to treat similar symptoms during illness. For example, Veratrum album (white hellebore), which was considered effective against cholera, caused violent purging that led to severe dehydration if administered in large doses—symptoms exactly like those of cholera itself. Between the 1st and 5th centuries CE the Romans made further developments in medicine. They introduced more herbs into the pharmacopeias, improved public hygiene, and observed the structure and function of the human body, although this was limited by social taboo, which prevented the dissection of bodies. Existing medical knowledge was codified and rationalized by Galen (?130–?200 CE), a Roman physician, anatomist, and physiologist. He adopted many ancient Greek principles, including the Aristotelian theory of the “four humors,” which claimed that the human body was made up of four humors—blood, choler (yellow bile), melancholy (black bile), and phlegm— that must be kept in balance to ensure vitality and health. After the decline of the Roman empire, little progress was made for centuries in the field of European medicine. A combination of herbal folklore, religious influences, and Galenic theory provided the basis for understanding and treating illness right through to the 17th century. Only when the Swiss physician and alchemist Paracelsus (1493–1541) began to develop his theories did the study of medicine start to evolve again. Paracelsus revived the ancient Greek theory of the Doctrine of Signatures, which was based on the premise that the external appearance of a plant—God’s “signature”—indicated the nature of its healing properties. For example, Chelidonium majus (greater celandine) was used to treat conditions affecting the liver and gallbladder because the yellow juice of the plant resembled bile. Paracelsus argued that disease was linked to external factors such as contaminated food and water rather than to mystical forces, and he challenged his contemporaries to recognize the body’s natural ability to heal itself, claiming that the practice of medicine should be based on detailed observation and “profound knowledge of nature and her works.” According to his theories, all plants and metals contained active ingredients that could be prescribed to match specific illnesses. Concentrating on practical experiments rather than on alchemy, he laid the foundations for the early stages of chemistry and subsequent development of pharmaceutical medicine, introducing new medicines, such as opium, sulfur, iron, and arsenic, into the contemporary repertory. His exploration of the chemical and medicinal properties of many substances, and his advocacy of the Hippocratic concept of “like cures like,” also made Paracelsus a key figure in the development of homeopathy. According to the British homeopath James Compton Burnett (1840–1901), the author of several important works on homeopathy that are still in use today, “Paracelsus planted the acorn from which the mighty oak of homeopathy has grown.”