The Middle Ages
The decline of ancient civilization in the West was severe. Although technology continued to develop (the horse collar, the stirrup, and the heavy plow came in), intellectual pursuits, including political philosophy, became elementary. In the Byzantine Empire, on the other hand, committees of jurists working for the emperor Justinian (reigned 527–565) produced the Codex constitutionum; the Digesta, or Pandectae; the Institutiones, which defined and condensed Roman law; and the Novellae consitutiones post codicem; the four books are collectively known as the Codex Justinianeus, or Code of Justinian. The Byzantine basileus, or autocrat, had moral responsibility for guarding and harmonizing an elaborate state, a “colony” of heaven in which reason and not mere will ought to rule. This autocracy and the orthodox form of Christianity were inherited by the Christianized rulers of the Balkans, of Kievan Russia, and of Muscovy.
In the West, two essential principles of Hellenic and Christian political philosophy were transmitted, if only in elementary definitions, in rudimentary encyclopaedias. St. Isidore of Sevilla, in his 7th-century Etymologiae (“Etymologies”), for example, asserts that kings rule only on condition of doing right and that their rule reflects a Ciceronic law of nature “common to all people and mankind everywhere by natural instinct.” Further, the Germanic tribes respected the civilization they took over and exploited; when converted, they revered the papacy. In 800 the Frankish ruler Charlemagne established a western European empire that would eventually be called holy and Roman (see Holy Roman Empire). The idea of a Christian empire coterminous with civilization thus survived in Western as well as Eastern Christendom.