source of illumination , on the grounds that both were disruptive of social harmony; they pleaded the cause of reason , in the hope that it could become the foundation of all human knowledge . Yet , for all that , they ma intained that a certain sort of mystical illumination lay at the heart of all true thought , and that human reason had validity only in virtue of i t s divine origin . They debated with Des cartes and took a keen interest in his mech- ism and his dualism ; they brought the atomistic theories of Democritus back into repute; and they sought to provide a detailed account of the causality link ing all phenomena.