 A century before al-Kindī, a different style of philosophizing had grown up in the Islamic world:  kalām.[12]  It has, indeed, been suggested that falsafa originated as a rival to kalām—claiming to be a better way of explaining the world than it could offer. Kalām was based on problems suggested by the Quran, but it was also inspired by the need to defend Islamic doctrine against philosophically based attacks by Christians and may have been informed by some ideas from the ancient Greek tradition. The dominant school of kalām in the ninth century were the Muʿtazilites. They were very confident in the ability of human reason to reach moral truth, to which, they held, God’s justice must  correspond.[13]  They strongly emphasized God’s unity and, in common with other kalām thinkers, developed an atomistic, materialistic physics, very different from Aristotle’s picture of a stable world of substances belonging to natural kinds. A Jewish version of kalām is found in the work of  Saadya  (882–942).[14]