The primacy of the Bishop of Rome is an ecclesiastical doctrine concerning the respect and authority that is due to the pope from other bishops and their sees.  Aidan Nichols wrote that "at root, only one issue of substance divides the Orthodox and the Catholic Churches, and that is the issue of the primacy." wrote that together with the Filioque controversy, differences in interpretation of this doctrine have been and remain the primary causes of schism between the Roman Catholic Church and the Orthodox Church. In the Eastern Orthodox Churches, some understand the primacy of the Bishop of Rome to be merely one of greater honour, regarding him as ("first among equals"), without effective power over other churches. Other Orthodox Christian theologians, however, view primacy as authoritative power: the expression, manifestation and realization in one bishop of the power of all the bishops and of the unity of the Church. The Roman Catholic Church attributes to the primacy of the Pope "full, supreme, and universal power over the whole Church, a power which he can always exercise unhindered", a power that it attributes also to the entire body of the bishops united with the pope. The power that it attributes to the pope's primatial authority has limitations that are official, legal, dogmatic, and practical.