Constantine of Kostenets (Bulgarian: Константин Костенечки, translit. Konstantin Kostenechki; born ca. 1380, died after 1431), also known as Constantine the Philosopher (Serbian: Константин Филозоф), was a medieval Bulgarian scholar, writer and chronicler, who spent most of his life in the Serbian Despotate. He is best known for his biography of Serbian despot Stefan Lazarević, which George Ostrogorsky described as "the most important historical work of old Serbian literature", and for writing the first Serbian philological study, Skazanije o pismenah (A History on the Letters). He was following the writing style of the Old Serbian Vita, first made popular in the Serbian scriptoriums of the 12th century.
Constantine was born in Bulgaria, probably in the town of Kostenets. In his youth, he attended school in Veliko Tarnovo, Bulgaria, and was taught by Andronik, a pupil of Patriarch Evtimiy of Bulgaria. He continued his studies on Mount Athos and in Constantinople. The Ottoman conquest of Tarnovo in 1393 drove him away and he settled in Stefan Lazarević's Serbian Despotate, probably around 1402. He was warmly welcomed by the Despot, also a man of letters and a benefactor of education, and was given the position of educator at his palace in Belgrade. Constantine also frequented the Manasija monastery, where he helped establish the Serbian "Resava School" of literature. His high education, life experience and traveling earned him the nickname of "Filozof" (Philosopher), after Saint Cyril the Philosopher. On top of the travels in his youth, he traveled to the Holy Land and, judging by his description of three missions to the palaces of eastern rulers (Timur, Musa and Mehmed I), he may also have participated.
Constantine's work had a tremendous impact on medieval Serbian literature and education. He introduced many classical Greek elements of literature and philosophy. His frequent citing of ancient philosophers and comparisons of the Despot in the Biography caused many to consider him a precursor to the Renaissance which, due to Ottoman conquest, never occurred in Serbian culture.
After Despot Stefan's death in 1427, the Serbian Patriarch Nikon ordered Constantine to write the Despot's biography. That order was only fulfilled four years later, after Stefan himself allegedly appeared in Constantine's dream and restated Nikon's order. The biography is one of the most interesting in the old Serbian literature because it contains not only facts about the Despot's life, but also geographic information and thorough descriptions of numerous historical events. He was apparently inspired by the imperial chronicles of Byzantine historians. The Biography of Despot Stefan Lazarević (Житија деспота Стефана Лазаревића) begins with a geographic description of Serbia's natural beauties, going on to describe its residents, praising their character but also mourning their forthcoming fall to the Turks. An exhaustive story of court events and the Despot's life follows, with numerous Biblical and classical references, as well with numerous historical data which have proven invaluable to later historians. On several occasions, Constantine used acrostics, with three masterpiece instances: in the introduction verses, in the titles of central chapters, and finally, in the verses telling of his sorrow for the deceased Despot.
Constantine of Kostenets, Pachomius the Serb, Cyprian, Metropolitan of Kiev, and Gregory Tsamblak were able to continue their literary activities virtually unhampered by any linguistic barrier when moving from the Balkans to Imperial Russia, no different than the role of Latin literature in the Roman Catholic part of medieval Europe. Constantine spoke and write a language which could not be identified with either the Bulgarian or Serbian vernaculars. Therefore, it was only natural for medieval Serbian biographers to turn for models to the body of existing vitae written in either Slavonic-Serbian or Church Slavonic language.