The validity of an imperial rescript constitutes one of the important problems of the legal sources of the postclassical period. The classical imperial rescript was used as a form of an imperial interpretation of the existing body of law. It could not be issued arbitrarily or at will by the emperor, but only in response to a petition of a particular person. The emperor could not interfere with private matters of private individuals. The situation changed, however, in the postclassical period. The imperial chancery introduced the notion of «spontaneus motus» to identify the possibility for the emperor to issue his enactments at will, first in regard to imperial edicts and leges generates (C.J. I. 14. 3), that even in the 4th century AD covered for the most part issues of public law. Under the emperor Justinian, however, the concept of spontaneus motus was extended unto imperial rescripts (C.J. I. 14. 12. 3). Now the imperial interpretation of existing body of law could be produced irrespectively to any particular case. This constituted, on the one hand, an arbitrary interference of the emperor with issues of private law, and on the other hand was a shift in respect to the interpretative functions of a rescript.