In medieval medical teaching, anatomy was taught by a professor reading Galen's text from a high chair while a barber-surgeon opened the body below and students tried to match the two. The authority lay in the book; the corpse merely illustrated the text. The trouble was that Galen, who lived in the second century, had dissected not humans but mostly apes and pigs — and his errors had been held sacred for thirteen centuries.

The Fleming Andreas Vesalius (1514–1564), professor of anatomy at the University of Padua, inverted this order. He came down from the chair, took the knife into his own hand, and put observation before text. His seven-volume "De humani corporis fabrica" (On the Fabric of the Human Body), printed in Basel in 1543, quietly corrected more than two hundred of Galen's mistakes: the human jawbone is a single piece (Galen said two); there is no porous wall in the heart through which blood was supposed to seep.

What made the book epochal was not only its content but its form. The engravings from Titian's workshop (most likely Jan Stephan van Calcar) showed flayed figures standing in Italian landscapes as if alive; as the muscle layers were peeled back in sequence, the body could be read like a work of architecture. Thanks to the printing press these images were identical everywhere — the muscle a student saw in Padua matched exactly the one another saw in Paris.

Vesalius's real revolution was neither a bone nor a vessel but a stance: the source of knowledge is not the authority of the past but the object before you. In that same year of 1543 Copernicus published a comparable break for the heavens. A century later Harvey would show that blood circulates, three centuries later Pasteur would find the invisible agent of disease; all walked the road Vesalius opened, placing observation ahead of authority.