"But these are images that correspond to the signs of the zodiac and the mansions of the moon, familiar to many mnemonic systems," I said, hoping not to betray my unease. "They are popular because they are based on regular numerical divisions, which aids in recall, but in the end they are merely images."

"Nothing is merely an image to the magician," Bernard snapped back. "All are signs pointing to hidden realities, as your title implies. Especially not those images derived from the ancient astrology of the Egyptians, as Agrippa well knew, for he was quoting from his master, Hermes Trismegistus, who was condemned by Saint Augustine for summoning demons!"

His voice rose on this last word; a cold hand gripped the base of my spine. I drew myself up to answer, but before I could speak, Sophia Underhill drew her chair nearer to the table, looked directly at me, and asked, cutting off Florio in mid-sentence, "Who is Hermes Trismegistus?"

The company fell silent; all eyes turned to me.

"I have read passing reference to his name in works of philosophy," she continued, with an innocence I did not quite believe, "but I can find none of his books in our library here, and I don't have permission to enter the university libraries."

"Nor should you, since you are not a scholar," chided her father, looking around the table as if embarrassed by her boldness. "I permit you to improve your mind by reading in our library as long as you keep your studies to what is fit for a lady's understanding."

I felt he said this for the benefit of the company; Sophia appeared about to protest, but then swallowed her words into a petulant expression. Her mother tutted again loudly.

"You will find no works of Hermes the Thrice-Great in Oxford now," Bernard said in a sonorous voice, shaking his head. "Before, we had them-before the great purge of the libraries in '69. Translated out of the Greek by the Florentine Ficino a century ago, at the dying request of Cosimo de' Medici. You know Ficino's version, Doctor Bruno?"

"I have read Ficino's translation," I said. "But I have also read the original Greek manuscripts, though the collection is incomplete. The fifteenth book was lost. Do you read Greek, Doctor Bernard?"

Bernard fixed me with those bright, accusing eyes. "Yes, I read Greek, young man, we are not all barbarians north of the Tiber. But the missing book is a myth-it never existed," he added briskly. Then he went on, in a softer tone. "I read Ficino too, when I was young, and Agrippa. There was not such a fear of the ancient writers then. But so many books are lost to us now, carried away by the tides of reform. Centuries of learning burned to ashes…" He trailed off and it seemed he had travelled deep into memory.

"Doctor Bernard," said the rector, a warning note in his voice again, "you know very well that the Royal Commission of '69 was sent to seek out heretical books acquired in the old monastic times, lest they infect the minds of our young men with their unholy ideas-a danger we senior Fellows must guard against still. I am sure you would not wish to disagree with such a prohibition."

Bernard gave a short, croaking laugh. "Books prohibited to scholars? How then should men of learning sharpen their intellect, or learn to discern between truth and heresy? And do those who proscribe not have the wit to realise that forbidden books lure men more potently than the lewdest temptress?" Here he cast a sideways glance at Sophia. "Oh yes-but a forbidden book will always find its way in through the cracks and the mouse-holes, do you not know that, Rector? If one only knows where to look." He cackled to himself as if this were a great joke, and I noticed his fellow scholars shifted uncomfortably in their seats.

"What happened to the books that were purged from the libraries then?" I asked, perhaps too urgently, for my question seemed to provoke a sudden hostility from Bernard; his eyes narrowed and he pulled himself stiffly upright.

"It was a long time ago," he said brusquely. "Burned or taken away by the authorities, who knows? I am old now, and I have forgotten those days."

He did not quite meet my gaze and I knew he was lying; a man who spoke so passionately about books would surely have remembered a public bonfire of them, even if it was so many years ago. But if the forbidden books were not burned, they must have passed into someone's hands, and I wondered if the old man knew whose.

"Doctor Bruno, you have still not answered my question," Sophia cut in, leaning across to tap my hand while fixing me with her wide-set, tawny eyes. The hint of a smile played around her full mouth, as if she too knew a great joke and was considering letting us in on it. "Who was he?"

I took a deep breath and returned her expectant gaze as steadily as I could, aware that the whole table had fallen silent, awaiting my answer, and that there was every chance my next words might be considered blasphemy.

"Hermes Trismegistus, called the Thrice-Great, was an Egyptian high priest of great antiquity," I began, turning over a piece of bread in my fingers. "He lived after the time of Moses, long before Plato or Christ. Some say he was the Egyptian god Thoth, the divinity of wisdom. In any case, he was a man of unusual insight who achieved, through profound contemplation of the cosmos and experimentation with the properties of the natural world, the wisdom to unlock the secrets written in the book of nature and the heavens. He claimed to have entered and understood the Divine Mind." I paused. "He claimed he could become equal to God."

There was a collective gasp from around the table; these men knew that this was indeed dangerous ground to tread, and I quickly added, "He is called the first philosopher, the first theologian, and he was also a prophet-Lactantius credited him with foretelling the advent of the Christian faith, in the very words of the Gospel."

"And Augustine said he had his foreknowledge from the Devil," Roger Mercer said eagerly, his face reddening further as half-chewed meat fell from his mouth and lodged in his beard, though he appeared not to notice. "For does Hermes not write of how the Egyptians animated the idols of their gods in magical rites by calling down the powers of demons?"

"I have never believed the account of the demons and the statues," I said lightly. "Men have always created mechanical toys and automatons and claimed to have endowed them with the gift of life, like the brazen head possessed by Roger Bacon that was reported to prophecy. But this is merely conjuring and skilled craftsmanship."

"Hermes Trismegistus was no magician, then?" Sophia said softly, still looking at me. She seemed disappointed.

"He wrote at length on the hidden properties of plants and stones and the arrangement of the cosmos," I replied. "There are some who call this alchemy or natural magic, and others who call it scientific enquiry."

"When it is done for the purpose of seeking forbidden powers, it is called sorcery," the rector put in, sternly.

"But did he discover any magic that worked?" she persisted, ignoring her father.

"How do you mean, worked?" I asked.

"I mean, was he able to use this natural magic to influence the world-to change people's thoughts or deeds, for example, and did he write of how it is done?" Her eyes were bright and impatient now as she leaned closer.

"Recipes for spells, you mean?" I laughed. "I'm afraid not. The Hermetic magic, if you want to give it that name, is concerned with teaching the adept how to penetrate the mysteries of the universe through the light of the intellect. He cannot teach you how to make your sweetheart fall in love with you or keep him true-for that you had better consult some village wisewoman."

There was some amusement at this from those at our end of the table, but the girl coloured violently and I suspected that my joke had accidentally struck the truth, so to cover her embarrassment I continued, hastily. "But the German alchemist Henry Cornelius Agrippa does speak of such things, in his treatise on the occult sciences that Doctor Mercer mentioned earlier. He writes that as well as the celestial images used in magic, we may create our own fitting to our purpose. For example, he says that to procure love, we may create an image of people embracing."