Soul (نَفْس, *nafs*) is a spiritual, immaterial substance. Influenced by Aristotle, Neoplatonism, and Qur’anic revelation. Avicenna: soul is simple, incorporeal, and individuated at creation. Soul progresses through stages: vegetative, animal, rational. Mystical traditions (e.g., Sufism) focus on purification of the soul. Philosophical arguments for immortality and resurrection abound.