The conceptions of life and the world which we call 'philosophical' are a product of two factors: one, inherited religious and ethical conceptions; the other, the sort of investigation which may be called 'scientific'. Philosophy, as distinct from theology, began in Greece in the sixth century B.C. After running its course in antiquity, it was again submerged by theology as Christianity rose and Rome fell. Its second great period, from the eleventh to the fourteenth centuries, was dominated by the Catholic Church, except for a few great rebels, such as the Emperor Frederick II. During the period of darkness, from the end of the fifth century to the middle of the eleventh, the western Roman world underwent some very interesting changes. The conflict between duty to God and duty to the State, which Christianity had introduced, took the form of a conflict between Church and king.