The development of European thought as we know it from the dawn of history down to the Dark Ages is marked by the successive secularization and de-secularization of knowledge.[1] From the beginning Greek secular science can be seen painfully disengaging itself from superstition. For some centuries it succeeded in maintaining its separate existence and made wonderful advances; then it was obliged to give way before a new and stronger set of superstitions which may be roughly called Oriental. In the following centuries all those branches of thought which had separated themselves from superstition again returned completely to its cover; knowledge was completely de-secularized, the final influence in this process being the victory of Neoplatonized Christianity.[2] The sciences disappeared as living realities, their names and a few lifeless and scattered fragments being all that remained. They did not reappear as realities until the medieval period ended. [1] _Cf._ S. Reinach, _Orpheus_, p. 36. [2] Neoplatonism, the last phase in the decline of ancient philosophy, profoundly influenced the Christian philosophy of patristic and medieval times, for which it prepared the way. The “first principle” of this philosophy was “the supra-rational, that which lies beyond reason and beyond reality.” It was from this source that Christian mysticism and contempt for empirical knowledge were largely drawn. It has been said that Catholic Christianity “conquered Neoplatonism after it had assimilated nearly everything that it possessed.” Its influence was far greater in the eastern than in the western empire. See Harnack, _History of Dogma_, vol. i, App. 3, for a brief account of Neoplatonism. See also _Encycl. Brit._, 11th edition, Art. “Neoplatonism.” This process of de-secularization was marked by two leading characteristics; on the one hand, by the loss of that contact with physical reality through systematic observation which alone had given life to Greek natural science, and on the other, by a concentration of attention upon what were believed to be the superior realities of the spiritual world. The consideration of these latter became so intense, so detailed and systematic, that there was little energy left among thinking men for anything else. At the point where this de-secularizing process was complete, at the opening of the seventh century, lived the Spanish bishop and scholar, Isidore of Seville. His many writings, and especially his great encyclopedia, the _Etymologies_, are among the most important sources for the history of intellectual culture in the early middle ages, since in them are gathered together and summed up all such dead remnants of secular learning as had not been absolutely rejected by the superstition of his own and earlier ages; they furnish, so to speak, a cross-section of the debris of scientific thought at the point where it is most artificial and unreal.