Galen (Aelius or Iulius Galenus of Pergamum), a doctor and philosopher, was the son of a rich architect. Born in modern-day Bergama in western Turkey, he was introduced as a student to all the main philosophical theories of classical antiquity. On his own admission, this led him only into a confusion from which he was rescued by considering mathematics, which henceforth provided him with a paradigm for understanding truth and falsehood. From 145, following the appearance of the healing god Asclepius to his father in a dream, he turned to medicine. He sat at the feet of medical teachers in Pergamum, Smyrna, and Alexandria, as part of what is the longest recorded medical education from the ancient world. In 157 he returned to Pergamum as doctor to the gladiators of the high priest, but in 162 he traveled to Rome, the imperial capital. There he...