Synesius (Greek: Συνέσιος; c. 373 – c. 414), a Greek bishop of Ptolemais in the Libyan Pentapolis after 410, was born of wealthy parents, who claimed descent from Spartan kings, at Balagrae (Bayda now)  near Cyrene between 370 and 375.
In 398 he was chosen as an envoy to the imperial court in Constantinople by Cyrene and the whole Pentapolis. He went to the capital in occasion of the delivery of the aurum coronarium and his task was to obtain tax remissions for his country. In Constantinople he obtained the patronage of the powerful praetorian prefect Aurelianus. Synesius composed and addressed to Emperor Arcadius a speech entitled De regno, full of topical advice as to the studies of a wise ruler, but also containing a bold statement that the emperor's first priority must be a war on corruption and a war on the interpenetration of barbarians into the Byzantine army.
His three years' stay in Constantinople was wearisome and otherwise disagreeable; the leisure it forced upon him he devoted in part to literary composition. Aurelianus succeeded in granting him the tax remission for Cyrene and the Pentapolis and the exemption from curial obligations for him, but then he fell in disgrace and Synesius lost everything. Later Aurelian returned in power, restoring his own grants to Synesius. The poet, then, composed Aegyptus sive de providentia, an allegory in which the good Osiris and the evil Typhon, who represent Aurelian and the Goth Gainas (ministers under Arcadius), strive for mastery, and the question of the divine permission of evil is handled.
In 402, during an earthquake, Synesius left Constantinople to return to Cyrene. Along the road he passed through Alexandria, where he returned in 403; it was in the Egyptian city that he married and lived, before returning at Cyrene in 405. The following years were busy for Synesius. His major concern was the organisation of the defence of the Pentapolis from the yearly attacks of neighbouring tribes.
In 410 Synesius, whose Christianity had until then been by no means very pronounced, was popularly chosen to be bishop of Ptolemais, and, after long hesitation on personal and doctrinal grounds, he ultimately accepted the office thus thrust upon him, being consecrated by Theophilus at Alexandria. One personal difficulty at least was obviated by his being allowed to retain his wife, to whom he was much attached; but as regarded orthodoxy he expressly stipulated for personal freedom to dissent on the questions of the soul's creation, a literal resurrection, and the final destruction of the world, while at the same time he agreed to make some concession to popular views in his public teaching.
His tenure of the bishopric was troubled not only by domestic bereavements (his three sons died, the first two in 411 and the third in 413) but also by the barbarian invasions of the country (in repelling which he proved himself a capable military organizer) and by conflicts with the praeses Andronicus, whom he excommunicated for interfering with the Church's right of asylum. The date of his death is unknown; it is usually given as c. 414, because he appears to have been unaware of the violent death of Hypatia.
His many-sided activity, as shown especially in his letters, and his loosely mediating position between Neoplatonism and Christianity, make him a subject of fascinating interest. His scientific interests are attested by his letter to Hypatia, in which occurs the earliest known reference to a hydrometer, and by a work on alchemy in the form of a commentary on pseudo-Democritus.
His extant works are:
- A speech before the emperor Arcadius, De regno (On Kingship)
- Dio, sive de suo ipsius instituto, in which he signifies his purpose to devote himself to true philosophy
- Encomium calvitii, a literary jeu d'esprit, suggested by Dio Chrysostom's Praise of Hair
- Aegyptus sive de providentia, in two parts, also known as The Egyptian Tale, about the war against the Goth Gainas and the conflict between the two brothers Aurelianus and Caesarius
- De insomniis, a treatise on dreams
- Catastasis, a description of the end of Roman Cyrenaica
- 159 Epistolae (letters, including one text -Letter 57- that is in fact a speech)
- 9 Hymni, of a contemplative, Neoplatonic character
- 2 homilies
- An essay on making an astrolabe
- A book on dog breeding
- Poems, mentioned in Synesius' letters
- Editio princeps, Turnebus (Paris, 1553)
- Antonio Garzya, (ed.), Opere di Sinesio di Cirene, Classici greci, Torino: UTET, 1989 (with Italian translation)
- Lacombrade, Garzya, and Lamoureux (eds.), Synésios de Cyrène, Collection Budé, 6 vols., 1978-2008 (with French translation by Lacombrade, Roques, and Aujoulat)
- T.D. Barnes, "Synesius in Constantinople," Greek, Roman, and Byzantine Studies 27 (1986): 93-112.
- A.J. Bregman, Synesius of Cyrene, Philosopher-Bishop (Berkeley, 1982).
- A. Cameron and J. Long, Barbarians and Politics at the Court of Arcadius (Berkeley, 1993).
- A. de Francisco Heredero, "Bárbaros en la Cirenaica a través dela obra de Sinesio de Cirene" in: D. Álvarez Jiménez, R. Sanz Serrano and D. Hernández de la Fuente, El Espejismo del bárbaro : ciudadanos y extranjeros al final de la Antigüedad (Castellón, 2013) 
- A. de Francisco Heredero, "Synesios of Cyrene and the Defence of Cyrenaica" in: A. de Francisco, D. Hernández y S. Torres (eds.), New Perspectives on Late Antiquity in the Eastern Roman Empire (Cambridge, 2014) 
- Chr. Lacombrade, Synesios de Cyrène. Hellène et Chrétien (1951)
- J. H. W. G. Liebeschuetz, Barbarians and Bishops: Army, Church and State in the Age of Arcadius and Chrysostom (Oxford 1990).
- Konstantinos D.S. Paidas, He thematike ton byzantinon "katoptron hegemonos" tes proimes kai meses byzantines periodoy (398-1085). Symbole sten politike theoria ton Byzantinon (Athens, 2005).
- ib., 'Why Did Synesius Become Bishop of Ptolemais?', Byzantion 56 (1986): 180-195.
- D. Roques, Etudes sur la correspondance de Synesios de Cyrene (Brussels, 1989).
- T. Schmitt, Die Bekehrung des Synesios von Kyrene (2001).
- Hartwin Brandt, "Die Rede peri basileias des Synesios von Kyrene - ein ungewoehnlicher Fuerstenspiegel," in Francois Chausson et Etienne Wolff (edd.), Consuetudinis amor. Fragments d'histoire romaine (IIe-VIe siecles) offerts a Jean-Pierre Callu (0Roma: "L'Erma" di Bretschneider, 2003) (Saggi di storia antica, 19), 57-70.
- Ilinca Tanaseanu-Doebler, Konversion zur Philosophie in der Spaetantike. Kaiser Julian und Synesios von Kyrene (Stuttgart, Steiner, 2005) (Potsdamer altertumswissenschaftliche Beitraege, 23).
- Dimitar Y. Dimitrov, "Sinesius of Cyrene and the Christian Neoplatonism: Patterns of Religious and Cultural Symbiosis," in Mostafa El-Abbadi and Omnia Fathallah (eds), What Happened to the Ancient Library of Alexandria? (Leiden, Brill, 2008) (Library of the Written Word, 3).
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "article name needed". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- Synesius is portrayed in Ki Longfellow's Flow Down Like Silver, Hypatia of Alexandria in a highly imaginative way.
- Heresy: the Life of Pelagius (2012) by David Lovejoy though fictional contains a portrait of Synesius based on his letters
- National Library of Australia
- Synesius, Epistulae 54,136.
- De regno 3; De insomniis 9; Hymns III.431.
- De regno 3.
- De providentiae 3.
- Konstantinos D.S. Paidas, He thematike ton byzantinon "katoptron hegemonos" tes proimes kai meses byzantines periodoy(398-1085). Symbole sten politike theoria ton Byzantinon, Athens 2005, passim.
- Epistulae, 31, 34, 38.
- Epistulae, 61.
- Epistulae, 4.
- Epistulae, 123, 129, 132.
- Epistulae, 105.
- "Synesius, Letter 015". www.livius.org. Retrieved 2016-10-28.
- Heresy: the Life of Pelagius by David Lovejoy, Echo Publications
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- Works written by or about Synesius at Wikisource
- Synesius of Cyrene at livius.org (introduction by Jona Lendering and translations of all epistles, all speeches, all hymns, both homilies, all treatises)
- D. Roques, Études sur la correspondance de Synésios de Cyrène reviewed by John Vanderspoel, University of Calgary, in Bryn Mawr Classical Review 02.01.16
- Cameron and Long reviewed by R.W. Burgess in BMCR
- Catholic Encyclopedia