Ctesias was born in Cnidus, a Greek city on the coast of Caria in what is now southwestern Turkey. Cnidus was famous for its medical school, one of the most advanced in the Greek world, and Ctesias trained there in the arts of healing that would one day carry him to the throne room of the Persian king. In the late fifth century BCE, when the Greek world was exhausted by the Peloponnesian War and the Persian Empire stood unchallenged from Egypt to the Indus, Ctesias travelled east. He entered the service of Artaxerxes II, the son of Darius II who had come to the throne in 404 BCE and who would rule for forty-six years. The Persian court was a world of unimaginable splendour. Susa and Babylon were cities of gold and glazed brick, where the king dined from golden plates and where messengers on the Royal Road could carry news from Sardis to Persepolis in a matter of days. Ctesias, as a Greek physician, held a respected position. He was not merely a doctor but an intellectual, a man who spoke with kings and generals and who had access to the archives and ambassadors of the greatest empire on earth.

His opportunity for fame came at the battle of Cunaxa in 401 BCE. Artaxerxes II fought his younger brother Cyrus the Younger, who had raised an army of Greek mercenaries and marched from Sardis to seize the throne. The battle was a confused affair, but Cyrus was killed and his Greek mercenaries, the Ten Thousand, began their famous retreat across Mesopotamia, a march later immortalised by Xenophon. Artaxerxes was wounded in the battle, and Ctesias treated him. The physician's skill saved the king's life, or at least his limb, and from that day Ctesias enjoyed royal favour. He remained at court for seventeen years, gathering material for the works that would make him famous.

The Persica was his first major work. In twenty-three books, Ctesias told the history of the Assyrian, Median, and Persian empires from the legendary Ninus and Semiramis to his own day. He claimed to have used Persian royal archives, the so-called cuneiform tablets and court records that no Greek had ever consulted before. His account differed sharply from Herodotus, the father of history, on almost every point. Where Herodotus said the Medes ruled before the Persians, Ctesias said the Assyrians ruled first, then the Medes, then the Persians. Where Herodotus made Cyrus the Great a Persian of humble birth, Ctesias made him the son of a Median princess and a bandit father. Where Herodotus described the Persian court in broad strokes, Ctesias gave intimate details of harem intrigues, poisonings, and palace coups that only an insider could know. Modern historians are divided on his reliability. Some of his details have been confirmed by archaeology and by Persian sources. Others are clearly romantic inventions, the gossip of a court physician who loved scandal more than truth.

But it was the Indica that made Ctesias immortal. India, in the fifth century BCE, was the farthest edge of the known world, a land of fabulous wealth and stranger customs that Persian traders reached by crossing the Hindu Kush. Ctesias never went to India himself, or if he did he never claimed the journey in the surviving fragments. Instead he gathered reports from Persian-speaking travellers who had visited the subcontinent, from Indian merchants at the Persian court, and from the gifts and curiosities that reached Susa from the east. He wrote of a land where giant ants, smaller than dogs but larger than foxes, dug gold from the earth and defended it so fiercely that the locals had to steal it by distracting them with camels. He described the skiapodes, one-legged men whose single foot was so large that they used it as a parasol against the Indian sun. He told of the cynocephali, dog-headed men who communicated by barking and lived in the mountains. He wrote of people whose ears were so enormous that they could wrap themselves in them like blankets. And he described the monoceros, a creature with the body of a horse, the head of a stag, the feet of an elephant, and a single horn on its forehead, the earliest European description of the unicorn.

The most famous creature in the Indica was the manticore. Ctesias described it as a beast the size of a lion, with red fur, a human face, three rows of teeth, and a scorpion tail that shot venomous stingers like arrows. He claimed to have seen one such creature brought to the Persian king as a living gift from India. The manticore entered European literature through his description and never left, migrating from natural history into medieval bestiaries, church carvings, and heraldic shields. Whether Ctesias actually saw a manticore, or a distorted tiger, or nothing at all, no one can say. What is certain is that his description was vivid enough to survive for two millennia.

The ancient world was divided on Ctesias' credibility. Aristotle dismissed his hybrid monsters as biological impossibilities, noting that animals of such different kinds could never breed with one another. The satirist Lucian, writing in the second century CE, mocked Ctesias as a liar who had never seen India but wrote about it as if he had lived there. The geographer Strabo criticised him for credulity. Yet other writers trusted him. Pliny the Elder quoted him extensively in his Natural History. Aelian used him as a source for his treatise on animals. Photius, the ninth-century patriarch of Constantinople, preserved a detailed summary of both the Persica and the Indica in his Bibliotheca, without which almost nothing of Ctesias would survive today. The Byzantine scholar Photius noted that Ctesias wrote in a simple, clear style, accessible to any reader, and that his stories were entertaining even when they strained belief.

Ctesias returned to Cnidus after his years at the Persian court, and there he died. The date of his death is unknown, but it was probably in the early fourth century BCE. He left behind no school of followers, no medical treatises that survived, and no historical method that later writers adopted. His works were lost within a few centuries of his death, surviving only in fragments and summaries. Yet his influence was profound. The creatures he described - the manticore, the unicorn, the giant ants, the dog-headed men - became fixtures of the European imagination. His Persica shaped Greek understanding of the Near East for generations. And his name became a symbol of the boundary between history and fable, the place where a physician's eye for detail meets a storyteller's love of the marvellous.