LEO of Thessalonica, surnamed the Philosopher, a learned ecclesiastic of the ninth century. The date and place of his birth are unknown. He studied grammar and poetry at Constantinople, and arithmetic, rhetoric, and philosophy in the island of Andros, under Michael Psellus. By exploring all the monastic libraries in continental Greece, and perusing their valuable contents, he amassed a large stock of general information, and, returning to Constantinople, became a teacher. One of his pupils having been captured in war by the Moslems, and becoming known through his geometrical acquirements to the caliph, who was a patron of learning, extolled the attainments of his master Leo so highly that he was despatched with a letter to the Philosopher inviting him to Baghdad. Afraid to correspond with an enemy, Leo contrived that the letter should be laid before the Emperor Theophilus, and was thus recommended to royal patronage. The emperor appointed him public teacher or professor, and afterwards ordered the Patriarch Joannes, the celebrated Iconoclast, and a kinsman of Leo's, to consecrate him Bishop of Thessalonica. On the death of the emperor, and the instalment of his widow Theodora in the regency, he was deposed from his see for his icono-
clastic opinions, but was afterwards appointed to the management of a mathematical school, established in Constantinople by the Cæsar Bardas. The latest period of Leo's life of which we have any notice is 869. The date of his death is unknown.
The only extant works that can with any probability be ascribed to Leo the Philosopher are several astrological MSS. existing in various European libraries.