Deep Spirits

Recent research suggests that much of his prophetic work paraphrases collections of ancient end-of-the-world prophecies (mainly Bible-based), supplemented with references to historical events and anthologies of omen reports, and then projects those into the future in part with the aid of comparative horoscopy. Hence the many predictions involving ancient figures such as Sulla, Gaius Marius, Nero, and others, as well as his descriptions of "battles in the clouds" and "frogs falling from the sky."

His historical sources include easily identifiable passages from Livy, Suetonius, Plutarch and other classical historians, as well as from medieval chroniclers such as Geoffrey of Villehardouin and Jean Froissart. Many of his astrological references are taken almost word for word from Richard Roussat's Livre de l'estat et mutations des temps of 1549–50.

One of his major prophetic sources was evidently the Mirabilis Liber of 1522, which contained a range of prophecies by Pseudo-Methodius, the Tiburtine Sibyl, Joachim of Fiore, Savonarola and others (his Preface contains 24 biblical quotations, all but two in the order used by Savonarola). This book had enjoyed considerable success in the 1520s, when it went through half a dozen editions (see External links below for facsimiles and translations) but did not sustain its influence, perhaps owing to its mostly Latin text, Gothic script and many difficult abbreviations.

Nostradamus was one of the first to re-paraphrase these prophecies in French, which may explain why they are credited to him. It should be noted that modern views of plagiarism did not apply in the 16th century. Authors frequently copied and paraphrased passages without acknowledgement, especially from the classics.The latest research suggests that he may in fact have used bibliomancy for this—randomly selecting a book of history or prophecy and taking his cue from whatever page it happened to fall open at.

Nostradamus's reliance on historical precedent is reflected in the fact that he explicitly rejected the label prophet (i.e. a person having prophetic powers of his own) on several occasions.