Western legal systems first recognized a core underpinning of conflict of laws—namely, that "foreign law, in appropriate instances, should be applied to foreign cases"—in the twelfth century. Prior to that, the prevailing system was that of "personal law", in which the laws applicable to each individual were dictated by the group to which he or she belonged. Initially, the mode of this body of law was simply to determine which jurisdiction's law would be most fair to apply; over time, however, the law came to favor more well-defined rules. These rules were systematically summarized by law professor Bartolus de Saxoferrato in the middle of the fourteenth century, a work that came to be cited repeatedly for the next several centuries.