Anonymus 603 was a monk to whom the patriarch Photius (Photios 1) wrote a letter between April 862 and April 866 as to one who had fallen into error; Anonymus 603 is said to have erred and to have remained in error, and to have abandoned the society of monks and angels for that of demons: Photius, Ep. 12 (I 64 Laourdas-Westerink). Anonymus 603 is addressed as μονάζοντι ἐκπεσόντι).

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