Forgery before AI: Querying the Pseudo-Isidorian Decretals with LLMs

The Pseudo-Isidorian Collectio Decretalium (c. 830–850) is the pre-print era’s most ambitious enterprise of forgery: a vast canonical collection interweaving authentic and fabricated material, from conciliar acts to papal letters. Its opening part consists of letters attributed to the thirty pre-Nicene bishops of Rome, from Clement I to Miltiades – fabricated in their entirety, stitched from thousands of authentic biblical, patristic and legal excerpts to sound credible to contemporaries and posterity. This section – the field of my current research – most fully lays bare the workshop of a forger who deceived readers for seven centuries. The corpus offers a unique laboratory for one question: how can machines that generate plausible text today assist in studying plausible text “generated” in the ninth century?

The paper demonstrates a pilot LLM-assisted source inventory on a sample of the corpus: automatic detection of biblical quotations and allusions, distinguishing Vulgate wording, Vetus Latina readings and borrowings mediated by the Fathers, alongside patristic and canonistic excerpts. Results are validated against Hinschius’s apparatus and Karl-Georg Schon’s Pseudoisidor transcriptions, measuring precision and recall and typologizing errors. I will show where the model outperforms the traditional toolkit (scale, unflagged quotations, paraphrase) and where it fails (hallucinated attributions, mistaking a quotation’s intermediary for its source).

The demonstration is paired with a hermeneutic reflection: medieval forgery as a mirror of today’s anxieties about synthetic text. Pseudo-Isidore proves that dangerously plausible text is no invention of LLMs – and philology has long possessed the tools for unmasking it, worth translating into standards for working with language models. I close by situating the method within a wider pre-print ecosystem – retrieval over editions, verification on MDZ scans, the Clavis Canonum – as a working model for the lone scholar without programming skills or grant infrastructure.