 Simone Weil (1909–1943) philosophized on thresholds and across borders. Her persistent desire for truth and justice led her to both elite academies and factory floors, political praxis and spiritual solitude. At different times she was an activist, a pacifist, a militant, a mystic, and an exile; but throughout, in her inquiry into reality and orientation to the good, she remained a philosopher. Her oeuvre features deliberate contradiction yet demonstrates remarkable clarity. It is value centered and integrated but not systematic. It contains scattered notes of her translations of and commentaries on several ancient Greek texts, Pythagorean geometry formulae, and detailed accounts of her daily tasks within a factory; but her oeuvre is also composed of addresses to political, industrial, and religious leaders as well as pieces intended for university students, radical militants, industrial workers, and farm laborers. In both her life and her thought—itself an unstable distinction with respect to Weil—she is a philosopher of margins and paradoxes. In part because Weil’s thought defies categorization, the ways in which her ideas are taken up often say as much about her commentator as they do about her. She was taken as a prototype for Albert Camus’s révoltés and praised by André Gide as “the patron saint of all outsiders”. Giorgio Agamben described her conscience as “the most lucid of our times”, and Hannah Arendt claimed that perhaps only Weil treated the subject of labor “without prejudice and sentimentality”. Maurice Blanchot described Weil as an “exceptional figure” who offers “an example of certitude” in the modern world, and Iris Murdoch wrote of “a profoundly disciplined life behind her writings” that gave “an authority which cannot be imitated”. But Weil was also criticized by Leon Trotsky as a “melancholy revolutionary” and disparaged as “crazy” by Charles de Gaulle. These remarks, however, betray an irony of which Weil was well aware and about which she was deeply concerned near the end of her life, namely, that her person would be considered more than her thought. By categorically focusing on the philosophical concepts Weil articulated and developed, this entry presents her philosophy while speaking to her concern.