The influence of the Spanish Jesuit Francisco Suarez (1548-1617) on 17th-century philosophy, theology, and law can hardly be underestimated. In this groundbreaking book, Daniel D. Novotny explores one of the most controversial topics of Suarez's philosophy: "beings of reason." Beings of reason are impossible intentional objects, such as blindness and square-circle. The first part of this book is structured around a close reading of Suarez's main text on the subject, namely Disputation 54. The second part centres on texts on this topic by other outstanding philosophers of the time, such as the Spanish Jesuit Pedro Hurtado de Mendoza (1578-1641), the Italian Franciscan Bartolomeo Mastri (1602-73), and the Spanish-Bohemian-Luxembourgian polymath Juan Caramuel de Lobkowitz (1606-82). The book should be of interest not just to those concerned with beings of reason but also for all those with a broader interest in the history of the period. It is written in a clear style that will make it appealing both to historians of philosophy and to anyone interested in applying analytical tools to the history of philosophy.