After completing her Master of Arts in philosophy, Sontag began doctoral research in metaphysics, ethics, Greek philosophy, Continental philosophy, and theology at Harvard.[13] The philosopher Herbert Marcuse lived with Sontag and Rieff for a year while working on his 1955 book Eros and Civilization.[14]: 38  Sontag researched for Rieff's 1959 study Freud: The Mind of the Moralist before their divorce in 1958, and contributed to the book to such an extent that she has been considered an unofficial co-author.[15] The couple had a son, David Rieff, who went on to be his mother's editor at Farrar, Straus and Giroux, as well as a writer in his own right. According to Sontag's biographer Benjamin Moser, Sontag was the true author of the text on Freud, which she wrote after David's birth, and in the separation the latter was the subject of an exchange: she handed over the authorship of the book to Rieff, he gave her their son.[16]