Greek theologian and exponent of Hesychasm. He was a monk of Mt Athos. In 1337 he became involved in controversy with Barlaam, a monk from Calabria, who stated the doctrine of God's unknowability in an extreme form. In answer to Barlaam's attack on the contemplative practices of the Hesychasts, Gregory wrote the Triads in defence of the Holy Hesychasts. He later produced a more succinct exposition of his theology, the One Hundred and Fifty Chapters. He argued that the physical exercises used by the Hesychasts in prayer, as well as their claim to see the Divine Light with their bodily eyes, could be defended in virtue of the biblical notion of the human as a single whole, body and soul together. In 1347 he was consecrated Abp. of Thessalonica. Feast day in the E., 14 Nov.; also the second Sunday in Lent.