Leo the Mathematician or the Philosopher (Greek: Λέων ο Μαθηματικός or Φιλόσοφος, Leon ho Mathematikos or Philosophos; c.790 – after 869) was a Byzantine philosopher and logician associated with the Macedonian Renaissance and the end of the Second Byzantine Iconoclasm. His only preserved writings are some notes contained in manuscripts of Plato’s dialogues. He has been called a “true Renaissance man” and “the cleverest man in Byzantium in the 9th century”.He was archbishop of Thessalonica and later became the head of the Magnaura School of philosophy in Constantinople, where he taught Aristotelian logic.
Leo was born in Thessaly, a cousin of the Patriarch of Constantinople, John the Grammarian. In his youth he was educated at Constantinople, but he travelled to the monasteries of Andros, where he could obtain rare manuscripts and was taught mathematics by an old monk. He originally taught privately in obscurity in Constantinople. The story goes that when one of his students was captured during the Byzantine–Arab Wars, the Caliph al-Mamun was so impressed by his knowledge of mathematics that he offered Leo great riches to come to Baghdad. Leo took the letter from the caliph to the Byzantine emperor Theophilos, who, impressed by his international repute, conferred on him a school (ekpaideuterion) in either the Magnaura or the church of the Forty Martyrs.
In the version of the story recorded by Theophanes Continuatus, the caliph, upon receiving Leo’s letter of refusal, sent a letter requesting answers to some difficult questions of geometry and astrology, which Leo obliged. Al-Mamun then offered two thousands pounds of gold and a perpetual peace to Theophilos, if only he could borrow Leo’s services briefly; the request was declined. The emperor then honoured Leo by having John the Grammarian consecrate him metropolitan of Thessalonica, which post he held from the spring of 840 to 843. There is a discrepancy in this account, however, in that the caliph died in 833. It has been suggested that either the connection between the caliph’s final letter and Leo’s appointment as metropolitan is in error, or the caliph in question was actually al-Mutasim. This latter option squares with the account of Symeon the Logothete, who makes Leo teach at the Magnaura from late 838 to early 840 and was paid handsomely.