Why a 17th-century physician thought animal blood could cure human madness

Ideas & Philosophy · Published 3 October 2026 Swipe more like this →

In short: In 1667, Jean-Baptiste Denys pioneered blood transfusion by injecting sheep blood into humans. He believed that the perceived purity of animals could replace the corrupted blood of his patients. While his first subjects miraculously survived, his attempt to cure a mentally ill man ended in a fatal, historic scandal.

Jean-Baptiste Denys, a physician to King Louis XIV, operated during a period of intense scientific competition between France and England. In the 1660s, European researchers were obsessed with the potential of blood transfusion. Denys, seeking to establish his reputation, moved beyond animal experiments to human subjects. His theory was rooted in the belief that an animal’s temperament—such as a lamb’s perceived purity—could be transferred to a human through its blood, effectively cleansing the recipient of physical or mental ailments.

On June 15, 1667, Denys performed the first documented human blood transfusion by injecting about twelve ounces of lamb blood into a 15-year-old boy suffering from persistent fevers. The boy survived, likely because the volume of blood was small enough to avoid a lethal immune reaction. Encouraged by this apparent success, Denys turned his attention to Antoine Mauroy, a well-known man in Paris suffering from mental illness. Denys hoped that replacing Mauroy’s 'bad' blood with 'innocent' calf blood would restore his sanity.

The experiment on Mauroy proved disastrous. After receiving calf blood, Mauroy suffered from profuse sweating and burning sensations—now understood as a hemolytic reaction where the body’s white blood cells attack the foreign blood. Although Mauroy died, medical historian Holly Tucker notes that the actual cause was likely arsenic poisoning administered by competing surgeons. Regardless, the death led to a trial where Denys was cleared of responsibility, but the French court subsequently banned the practice. It would take until the 19th century for the scientific community to revisit transfusion, long before the 1900 discovery of human blood groups made the procedure safe.

The legacy of Denys is a cautionary tale about the intersection of early medical ambition and moral philosophy. While he is credited with documenting the first signs of a hemolytic transfusion reaction, his work was driven by a misunderstanding of biology. By attempting to treat the soul and the mind through the physical properties of animal blood, Denys inadvertently highlighted the extreme dangers of xenotransfusion, ultimately stalling medical progress for over a century.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• Can the vitality of youth be transferred through a simple blood transfusion?
• How a military surgeon turned blood serum into a weapon against deadly childhood diseases.
• Doctors kept bleeding patients for two centuries after Harvey disproved the idea
• How the horrors of the First World War transformed blood into a storable resource
• Insulin, the polio vaccine and diphtheria antitoxin all began with animal experiments
• Why Jean Baudrillard argued that the Gulf War never actually took place
• The last Maharaja of Mysore was a philosopher-king who bankrolled global musical history.
• Jean Bodin: The Renaissance thinker who defined sovereignty and hunted witches
• Why the internet's collective wisdom might be making us less intelligent and more isolated.
• The medieval philosopher who paved the way for modern physics
• The Polish pediatrician who walked into a death camp to protect his children
• The French philosopher who wrote his greatest work while imprisoned by the enemy

More in Philosophers · More in Ideas & Philosophy · Latest · A–Z · All topics