A popular picture has the forbidden books of antiquity locked in a monastery crypt, guarded by monks who knew what they had and told no one. It makes a good novel. The real story is less dramatic and more interesting, because it shows how fragile the chain was and how ordinary were the hands that held it.
What a monastic library was
For most of the early Middle Ages a monastery's books were few. A well-stocked house of the ninth century might own a few hundred volumes, and most owned far fewer. They were kept in a cupboard, the armarium, often in the cloister, under the care of an officer called the armarius, who commonly also directed the chant. Separate library rooms, with books chained to sloping desks, became usual only in the later Middle Ages.
Copying was slow and parchment was costly. A scriptorium copied first what the community needed: the Bible, the liturgy, the church fathers, and grammar. Pagan authors were copied because Latin could not be learned without them, and because some monks loved them. Whatever was not recopied in each generation was at risk, and a great deal was lost. Now and then a text no longer wanted was scraped away so that its parchment could be used again, leaving the ghost of the older writing underneath. Such a book is called a palimpsest.
The Asclepius, hidden in plain sight
One Hermetic work never needed to be rediscovered. The Latin Asclepius travelled through the Middle Ages bound up with the philosophical works of Apuleius, to whom its translation was wrongly credited. Augustine had quoted it at length in The City of God in order to condemn its account of statues brought to life, and in doing so he made sure that every educated reader would know the name of Hermes.
In the twelfth century the philosophers of the cathedral schools read the Asclepius with admiration, as the testimony of a Pagan sage to the creator. It was not forbidden. It stood on the shelf with the other philosophers.
The Greek dialogues and Byzantium
The Greek treatises that we call the Corpus Hermeticum took a different road. They were preserved in the Greek-speaking East, where scholars went on reading ancient philosophy without a break. The eleventh-century philosopher Michael Psellus knew them and left a disapproving note on one passage. All the manuscripts that survive descend from Byzantine copies, and none is older than the fourteenth century.
About 1460 a monk named Leonardo of Pistoia, one of several agents collecting Greek books for Cosimo de' Medici, brought a manuscript of fourteen treatises from Macedonia to Florence. Cosimo had Marsilio Ficino translate it at once. The Latin was finished in 1463 and printed in 1471.
Notice what kind of survival this was: a single manuscript, lacking the last treatises of the collection, carried west a few years after the fall of Constantinople in 1453.
The Arabic road
The technical Hermetica came by a third route. Greek works on astrology, alchemy, and the properties of things had been translated into Arabic from the eighth century onward, and Arabic authors wrote many more under the name of Hermes. In the twelfth and thirteenth centuries Latin scholars went to Spain and Sicily to translate them. Hugh of Santalla put the Emerald Tablet into Latin. Robert of Chester translated the first alchemical book in 1144. At the court of Alfonso X of Castile in the 1250s, the great Arabic handbook of astral magic was turned into Castilian, and then into Latin as the Picatrix.
Many of these translators were clerics, and the books they made entered the libraries of clerics. That brings us back to the cloister.
Monks who owned magic
Medieval books of ritual and astral magic were written in Latin, borrowed the forms of the liturgy, and assumed a reader who knew the psalms and the calendar. Their readers were therefore mostly clergy: priests, friars, monks, and students in minor orders. One historian has called this world a clerical underworld.
Sometimes the evidence is a catalog. The late medieval catalog of one great English abbey lists some thirty volumes containing works of magic, given by its own monks and shelved with astronomy and medicine. Sometimes it is a confession. Early in the fourteenth century the French monk John of Morigny described how he had practiced the Ars Notoria in pursuit of learning, came to believe it demonic, and wrote a purified replacement, which was itself burned at Paris in 1323. And sometimes it is a scandal. Johannes Trithemius, a German abbot, built one of the finest libraries of his day, some two thousand volumes, and wrote the Steganographia among them. When a visitor reported what the book seemed to contain, his reputation never recovered.
Condemned and kept
Were these books forbidden? Often, yes. The Paris condemnation of 1277 named books of geomancy and necromancy among its targets, and individual texts were burned. But the Middle Ages had no machinery for removing a title from every shelf in Christendom, and the line between the lawful and the unlawful was honestly disputed. Astrology was needed for medicine and for the calendar. Natural magic was a branch of natural philosophy. A book on the images of the planets might be shelved as astronomy by one librarian and denounced as idolatry by another.
So the texts survived in the gap between rule and practice: copied by the curious, cataloged under harmless headings, bound into miscellanies beside sermons and herbals, and left alone.
What the story teaches
By 1614, when Isaac Casaubon showed that the Hermetic dialogues were written in the early Christian centuries and not in the age of Moses, they had been in print for nearly a century and a half and could no longer be lost. For a thousand years before that, each of them had depended on someone deciding that it was worth the parchment.
That is the lesson a small library takes from the monks. Survival is not a vault. It is a habit: one person at a time choosing to copy, to shelve, to catalog, and to keep the door open. We try to keep the habit.