Boris I of Bulgaria
His most important theological work is the "Amphilochia", a collection of some 300 questions and answers on difficult points in Scripture, addressed to Amphilochius, archbishop of Cyzicus. Other similar works are his treatise in four books against the Manichaeans and Paulicians, and his controversy with the Latins on the Procession of the Holy Spirit. Photios also addressed a long letter of theological advice to the newly converted Boris I of Bulgaria. Numerous other "Epistles" also survive.