Six hundred years separated them. Together, they are the reason medicine exists as a profession, a discipline, and an idea — and one of them was deeply, consequentially wrong for over a millennium.
"Father of Medicine." Geboren am the island of Cos. Credited with establishing medicine as a discipline separate from religion and philosophy. Associated with the Hippocratic Corpus — approximately 60 texts — and the Hippocratic Oath. First to argue that disease had natural, not supernatural, causes.
The most prolific medical writer of antiquity. Born in Pergamon (modern Turkey). Physician to Roman emperors including Marcus Aurelius. Wrote over 500 texts, of which roughly 300 survive. His anatomical and physiological theories dominated Western and Islamic medicine for 1,400 years.
| Category | Hippocrates | Galen |
|---|---|---|
| Era | Classical Greece, c. 460–370 BCE | Roman Empire, 129–216 CE |
| Key contribution | Removed medicine from religion; clinical observation | Systematic anatomy, physiology, pharmacology |
| Major works | Hippocratic Corpus (~60 texts, authored by school) | ~300 surviving texts including On the Natural Faculties |
| Influence duration | Foundational for all subsequent medicine | Dominant medical authority for ~1,400 years |
| Key error | Humoral theory (4 humors) | Blood not circulating; invisible heart pores |
| Legacy today | Hippocratic Oath, principle of clinical observation | Medical terminology, anatomical description |
In the 5th century BCE, disease was divine punishment. Epilepsy was "the sacred disease" — believed to be caused by gods or demons. Treatment meant prayer, sacrifice, and ritual. The suggestion that illness might have physical, observable, natural causes was not merely a medical hypothesis; it was a philosophical revolution and, to many, an act of impiety.
Hippocrates — or more precisely, the school that bears his name — made that suggestion systematically and defended it. The Hippocratic text "On the Sacred Disease" is one of the most important documents in intellectual history. It opens: "I am about to discuss the disease called 'sacred.' It is not, in my opinion, any more divine or more sacred than other diseases, but has a natural cause." The author goes on to argue that what appears divine is simply unknown — that epilepsy is a brain disorder, not a visitation from the gods. This is not merely clever. It is the founding statement of scientific medicine.
The Hippocratic Corpus, a collection of roughly 60 texts produced by the school of Cos over several generations, established the practices of clinical observation, prognosis, and the physician's ethical relationship with patients. The Hippocratic Oath — probably not written by Hippocrates himself — codified the principle that the physician's first obligation is to the patient's welfare. Versions of it are still administered in medical schools worldwide.
Galen of Pergamon was born six centuries after Hippocrates and operated in a very different world — the cosmopolitan, literate, aggressively curious Roman Empire. He was a physician-scholar who combined clinical practice (he treated gladiators in Pergamon, giving him unusual access to traumatic injury) with a prodigious output of writing and a ferocious confidence in his own conclusions. He claimed to have read everything that came before him and to have corrected it.
His ambition was total: to write the definitive account of the human body, its functions, its diseases, and their treatments. He largely succeeded — in the sense that the next 1,400 years believed him. His anatomy was based largely on dissections of Barbary macaques (human dissection was banned in Rome), and he extrapolated from animal anatomy to human structure. This worked, to a degree. And where it failed — as in his account of the heart and blood vessels — the failures persisted for over a millennium because no one felt they had the authority or the evidence to contradict Galen.
The paradox of Galen's legacy is this: his authority was so great that it became an obstacle to the very empirical medicine he claimed to practice. Galen insisted that physicians should observe and reason from evidence. But he was so thorough and so confident that his works became a substitute for observation. Medieval physicians who found discrepancies between Galen's anatomy and the bodies in front of them frequently concluded that the bodies, not Galen, were wrong.
His central error was his account of blood and the heart. Galen believed that blood was continuously manufactured in the liver from food and then consumed by the organs. It did not circulate — it was produced and used up. He believed that the heart's two sides communicated through invisible pores in the septum. Both claims were false. Andreas Vesalius, dissecting human cadavers in Padua in the 1530s and 1540s, showed that the septal pores did not exist. William Harvey, in 1628, demonstrated that blood circulates. In each case, Galen had been wrong for over a millennium. The damage done was not merely scientific: physicians for 14 centuries had treated patients according to theories that included bloodletting to balance the humors — a practice that killed many of them.
Both Hippocrates and Galen were transmitted to medieval Europe partly through Arabic translations and commentary. The great Islamic physicians — Ibn Sina (Avicenna), Ibn Rushd (Averroes), Al-Razi (Rhazes) — worked directly with Greek medical texts and in many cases improved upon them. Avicenna's Canon of Medicine synthesized Greek medicine, particularly Galen, with clinical experience gathered across the Islamic world. It was translated into Latin in the 12th century and became a standard medical textbook in European universities for 600 years.
In this transmission, Hippocrates functioned as a symbol and a principle — the founding authority who established that medicine should be rational and observational — while Galen provided the actual content: the anatomy, the physiology, the pharmacology. The two were understood as complementary: Hippocrates gave medicine its ethics and method; Galen gave it its substance.
When Andreas Vesalius published De humani corporis fabrica in 1543, he did not intend to destroy Galen. He trained as a Galenist and revered Galen's ambition. But dissecting human cadavers with fresh eyes — using condemned criminals and bodies stolen from gibbets — he found error after error. He documented over 200 corrections to Galen's anatomy. His illustrations, produced by artists from Titian's workshop, were the most accurate representations of the human body ever made.
The reaction was ferocious. Jacobus Sylvius, Vesalius's own teacher in Paris, called him a madman and a liar. The profession of medicine had staked its authority on Galen for so long that correcting him felt like dismantling medicine itself. It took another century — Harvey's circulation, Malpighi's capillaries, the invention of the microscope — before Galenic medicine was fully replaced. By then, Hippocrates had survived better: the principle of clinical observation had outlasted the specific theories it originally supported.
Hippocrates gave medicine its identity as a rational discipline distinct from religion, its ethical framework, and its commitment to observation over authority. Galen gave it its first comprehensive system — detailed enough to be taught, memorized, and applied across centuries and cultures. Medicine needed both: the founding principle and the working body of knowledge.
The deepest difference between them is their relationship to error. Hippocratic medicine was, in principle, revisable — it said: observe the patient, modify the theory. Galenic medicine, as it was received, was canonical — it said: Galen has observed; learn his conclusions. That shift from method to authority is the most consequential transformation in the history of medicine, and it cost lives at a scale that is difficult to calculate. The lesson is not that Galen was wrong to be systematic, but that any system, however brilliant, that discourages challenge is eventually lethal.
Hippocrates is credited with removing medicine from religion and superstition, establishing that disease has natural causes. The Hippocratic Corpus — approximately 60 texts — introduced clinical observation, prognosis, and the Hippocratic Oath. He is called the Father of Medicine.
Galen produced a comprehensive, systematic account of the human body and disease that was more detailed and authoritative than anything before it. His books became the medical curriculum in medieval Europe and the Islamic world. His authority was so great that questioning him was considered professionally disqualifying. Galen's errors went unchallenged until Vesalius in 1543.
Galen believed that blood was continuously produced by the liver and consumed by the body — that it did not circulate. This was wrong. William Harvey demonstrated blood circulation in 1628, nearly 1,400 years after Galen. Galen also believed that blood passed through invisible pores in the septum of the heart, which Vesalius showed did not exist.
Almost certainly not. The Hippocratic Oath is part of the Hippocratic Corpus, a collection of approximately 60 texts written by various authors from the school of Cos. Scholars believe Hippocrates wrote none of them directly — they are works of his school compiled over several generations.