Bahya ibn Paquda, the chief exponent of Jewish pietism, gave that ecumenical strand of thought and practice a markedly philosophical cast, preferring the intellectual to the fideistic side of pietist tradition, and embracing rationalism as the ally of faith rather than rejecting it as an enemy. Drawing selectively from Muslim as well as Jewish sources, Bahya’s spiritual vademecum, al-Hidaya ila fara’id al-qulub (The Book of Guidance to the Duties of the Heart), was widely studied ever since its composition, especially in its medieval Hebrew translation, and parts of it are even included in the liturgical meditations of the Ten Days of Penitence. In it, Bahya thematizes his materials carefully, using his own sense of the reasonable to structure pietism as a philosophical system, controlling the monistic penchant of mysticism and disciplining the ascetic tendencies of the devotional cast of mind. Maimonides found Bahya’s asceticism excessive and rejected Bahya’s related leanings towards predestinarianism and resignation; but he quietly adopted Bahya’s moral and intellectual interpretation of the mystic quest for unity with God, fell into step with his predilection for spiritual immortality as distinguished from bodily resurrection, echoed his affirmation of God’s absolute unity and simplicity, and concurred in his admiration for negative theology, the theology of divine transcendence.