Evolution of the Ius commune
Legista sine canonibus parum valet, canonista sine legibus nihil
A Civil Lawyer without a knowledge of canon law is worth little, a canon lawyer without a knowledge of Roman law is worth nothing.
Reasons why Roman law was rediscovered and taught
Codex Florentinus of the Digest
Justinian's Corpus iuris civilis (Digest, Code, Institutes, Authenticum)
Pepo (1075)
Irnerius (ca. 1100-1125)
The Four Doctors of Roman Law (Bulgarus [ca. 1100-1166], Martinus [ca. 1100-?], Jacobus [ca. 1110-1169], and Hugo [ca. 1105-ca. 1168])
Diet of Roncaglia 1158
EmperorFrederick I Barbarossa 1152-1190