As a historian he was innovative and modern, rejecting traditional methods of historical inquiry, he turned to the direct study of the documents. His Antigüedades de España brought to light many documents, including many from the archives of Cardeña, that had not until then been publicised. It was published in two volumes, the second at Madrid from 1719–21. He uncovered many forgeries that exposed traditional narrative of the origins of Castile and the unity of Spain. Posthumously at Madrid in 1729 was published his Ferreras convencido, a revision of the history of Ferreras.
