The Scientific Revolution soon inspired renewed study of the microcosm of the human body. For many centuries the ancient Greek physician Galen’s explanation of the body carried the same authority as Aristotle’s account of the universe. According to Galen, the body contained four humors: blood, phlegm, black bile, and yellow bile. Illness was believed to result from an imbalance of humors, which is why doctors frequently prescribed bloodletting to expel excess blood.

Swiss physician and alchemist Paracelsus (1493–1541) was an early proponent of the experimental method in medicine and pioneered the use of chemicals and drugs to address what he saw as chemical, rather than humoral, imbalances. Another experimentalist, Flemish physician Andreas Vesalius (1516–1564), studied anatomy by dissecting human bodies, often those of executed criminals. In 1543, the same year Copernicus published On the Revolutions, Vesalius issued his masterpiece, On the Structure of the Human Body. Its two hundred precise drawings revolutionized the understanding of human anatomy. The experimental approach also led English royal physician William Harvey (1578–1657) to discover the circulation of blood through the veins and arteries in 1628. Harvey was the first to explain that the heart worked like a pump and to explain the function of its muscles and valves.

Some decades later, Irishman Robert Boyle (1627–1691) helped found the modern science of chemistry. Following Paracelsus’s lead, he undertook experiments to discover the basic elements of nature, which he believed was composed of infinitely small atoms. Boyle was the first to create a vacuum, thus disproving Descartes’s belief that a vacuum could not exist in nature, and he discovered Boyle’s law (1662), which states that the pressure of a gas varies inversely with volume.