Praxagoras, a descendant of Asclepius, was a resident of Cos in Greece. He is believed to have been born around 340 BC, although some historians have placed his date of birth as late as the last third of the fourth century. His father, Nicharchus, was an eminent physician who, according to Galen, held the theory that the arteries contain only air, while the veins contain blood. This theory was later also expounded by Praxagoras.

Praxagoras, came to be regarded in antiquity as the most famous member (after Hippocrates, who was probably somewhat older than Praxagoras) of the Coan School. Galen ranked Praxagoras beside Hippocrates and Diocles among the famous physicians of the old school. Later medical historians classified Praxagoras as a follower of the logical or dogmatic school, which advocated rigid doctrine rather than investigation. (The dogmatists divided medical science into five branches: physiology, etiology, hygiene, semeiology, and therapeutics...