One of the earliest Greek philosophers, Thales of Miletus (c. 7th century BCE), maintained that the universe contained a creative force that he called physis, an early progenitor of the term physics; he also postulated that the world and all living things in it were made from water. Anaximander, a student of Thales, did not accept water as the only substance from which living things were derived; he believed that in addition to water, living things consisted of earth and a gaslike substance called apeiron, which could be divided into hot and cold. Various mixtures of those materials gave rise to the four elements: earth, air, fire, and water. Although he was one of the first to describe Earth as a sphere rather than as a flat plane, Anaximander proposed that life arose spontaneously in mud and that the first animals to emerge had been fishes covered with a spiny skin. The descendants of those fishes eventually left water and moved to dry land, where they gave rise to other animals by transmutation (the conversion of one form into another). Thus, an early evolutionary theory was formulated.
At Crotone in southern Italy, where an important school of natural philosophy was established by Pythagoras about 500 BCE, one of his students, Alcmaeon, investigated animal structure and described the difference between arteries and veins, discovered the optic nerve, and recognized the brain as the seat of the intellect. As a result of his studies of the development of the embryo, Alcmaeon may be considered the founder of embryology.
Although the Greek physician Hippocrates, who established a school of medicine on the Aegean island of Cos around 400 BCE, was not an investigator in the sense of Alcmaeon, he did recognize through observations of patients the complex interrelationships involved in the human body. He also contemplated the influence of environment on human nature and believed that sharply contrasting climates tended to produce a powerful type of inhabitant, whereas even, temperate climates were more conducive to indolence.
Hippocrates and his predecessors were concerned with the central philosophical question of how the cosmos and its inhabitants were created. Although they accepted the physis as the creative force, they differed with regard to the importance of the roles played by earth, air, fire, water, and other elements. Although Anaximenes, for example, who may have been a student of Anaximander, adhered to the then-popular precept that life originated in a mass of mud, he postulated that the actual creative force was to be found in the air and that it was influenced by the heat of the Sun. Members of the Hippocratic school also believed that all living bodies were made up of four humours—blood, black bile, phlegm, and yellow bile—which supposedly originated in the heart, the spleen, the brain, and the liver, respectively. An imbalance of the humours was thought to cause an individual to be sanguine, melancholy, phlegmatic, or choleric. These words persisted in the medical literature for centuries, a testament to the lengthy popularity of the idea of humoral influences. For centuries it was also believed that an imbalance in the humours was the cause of disease, a belief that resulted in the common practice of bloodletting to rid the body of excessive humours.