 Ramón Xirau wrote philosophy, literary criticism, and poetry, and combined them in enriching ways. Xirau wrote about the relationship between philosophy and poetry as two different forms of knowledge, and about the ontological nature of being human, understanding ser and estar as different categories. Among his best-known books are: Introducción a la historia de la filosofía (1964), Poesía y conocimiento (1979), and El tiempo vivido (1985).