Domenicus Scepticus is a doctoral thesis that presents in a single study the corpus of El Greco's manuscript annotations on the margins of the printed books which he owned and which are identifiable to date. The short review of the artist's library is followed by a discussion of the two surviving books, which, although they are inventoried among the painter's possessions and the frontispieces of which establish further Greco's ownership, are not confirmed to date as displaying Greco's own handwriting on the margins. These are Appian's Delle Guerre... dei Romani (1551) and Xenophon's Operae Graece (1516). A short unedited study of the history of the provenance of Xenophon's Operae incorporates additional unpublished data, such a handwritten letter, and builds a series of hypotheses that needs further investigation regarding the Philhellenic culture in Toledo and Madrid of the Golden Age. In the main body of the thesis, the total number of Greco's autograph annotations in Vasari's Vitae (1568) and in Vitruvius's Died Libri dell' Architettura (1556) is examined in English, while the unedited transcription and translation of the notes in Serlio's Architettura (1566) is introduced -annotations attributed to Greco in all probability. Each chapter is dedicated to a single book's marginalia, presented in thematic units, each point analysed individually and compared or contrasted with other marginal statements where appropriate. Domenicos the Enquirer mainly studied Classical texts and annotated art volumes, both manifestations of his intellectual quest, echo of Renaissance Humanism that celebrates the revival of Classical thought and the pursuit of the universality of knowledge of Vitruvian prototypes. The expression of his beliefs in the marginalia represent an understated heritage in the history of modern publishing, from which this thesis draws conclusions in the interest of formulating Greco's theory of art, reflections of his thought in writing.