The spectrum of historical interaction between Islamic philosophy and Islamic mysticism or Sufism has typically ranged from rapprochement and mutual influence in some cases to criticism and polemical debate in others. This chapter examines the cross-pollinations, conceptual borrowings, and critiques that have defined Islamic philosophy’s encounter with the Sufi tradition, from the mystical influence on Avicenna and Ibn Ṭufayl to the adoption and, at times, critical scrutiny of Avicennian theories by the late medieval school of philosophical Sufism associated with Ibn ʿArabī and his followers. Drawing on primary texts in Arabic, it traces debates between philosophers and Sufis over the limitations of rationalist metaphysics, the ontological status of universals, and the nature of time, as well as exploring the ways in which philosophical Sufism integrates key Fārābian and Avicennian ideas—including necessary and contingent existence, and the notion of conjunction with the universal active intellect—into its theoretical frameworks.