In 1950, Arendt became a naturalized citizen of the United States. The same year, she started seeing Martin Heidegger again, and had what the American Writer Adam Kirsch called a "quasi-romance," that lasted for two years, with the man who had previously been her mentor, Teacher, and lover. During this time, Arendt defended him against critics who noted his enthusiastic membership in the Nazi party. She portrayed Heidegger as a naïve man swept up by forces beyond his control, and pointed out that Heidegger's philosophy had nothing to do with National Socialism. She served as a visiting scholar at the University of Notre Dame, University of California, Berkeley, Princeton University, and Northwestern University. In 1959, she was named the first female lecturer at Princeton. She also taught at the University of Chicago from 1963 to 1967, where she was a member of the Committee on Social Thought; The New School in Manhattan where she taught as a university professor from 1967 until her death in 1975; Yale University, where she was a fellow, as well as the Center for Advanced Studies at Wesleyan University (1961–62, 1962–63).