Ptolemy I, surnamed Soter, son of Lagus, and founder of the Graeco-Egyptian dynasty, born near the court of Philip of Macedon in 367 B. C., died in Alexandria in 283. His mother ArsinoŽ had been a concubine of Philip, and many therefore supposed him to be his son. He was one of the principal generals of Alexander the Great in his Asiatic campaigns. After the death of Alexander in 323, he became governor of Egypt during the nominal reigns of Philip Arrhidaeus and Alexander IV., and the regency of Perdiccas. One of his first acts was to put to death Cleomenes, who as receiver general of tributes had amassed an enormous fortune, and was a partisan of Perdiccas. In 322 he annexed the city and province of Cyrene. To oppose Perdiccas, he leagued in 321 with Antigonus, Antipater, and Craterus. Perdiccas invaded Egypt, but Ptolemy defeated him and prevented him from crossing the Nile. Subsequently, when Perdiccas was murdered by his own soldiers, Ptolemy sent wine and provisions to the invading army, and so won them that they offered him the regency, which he declined. In 320 he seized upon Phoenicia and Coele-Syria, and it was probably during this expedition that he took possession of Jerusalem without opposition by attacking it on the sabbath.
To resist Antigonus, he formed a coalition in 316 with Seleucus, Cassander, and Lysimachus; and after a struggle of four years, during which he lost Phoenicia, peace was concluded (311). In 310 Ptolemy renewed hostilities under the pretext that Antigonus had violated the treaty by keeping his garrisons in the Greek cities of Asia Minor and the adjacent islands, and in the long war which followed he lost Cyprus by his defeat in the sea fight near Salamis in 306. Antigonus assumed the title of king, and Ptolemy followed his example. Demetrius, the son of Antigonus and conqueror of Salamis, now invaded Egypt, but, baffled at the banks of the Nile, turned his arms against Rhodes, which had refused to join in the attack. Ptolemy enabled it to hold out by furnishing troops and provisions, and out of gratitude the Rhodians gave him the title of saviour (Soter). The death of Antigonus at the battle of Ipsus in 301 terminated the war, and added Syria and Palestine to Ptolemy's dominions; and in 295 Cyprus was recovered. In 287 he was in league with Seleucus and Lysimachus against Demetrius, but the rest of his reign was peaceful.
He made Memphis his capital, took measures to promote the happiness of his Egyptian subjects, revived their ancient religious and political constitution, and restored to the priestly caste some of its former privileges. He showed equal toleration to the Jews and the Greeks, and great numbers of both, among them scholars of the greatest renown, were attracted to Alexandria. He laid the foundation of literary institutions, the most celebrated of which were a library and a museum, a kind of university whose professors and teachers were supported at the public expense. Ptolemy wrote a history of the wars of Alexander. He wished his youngest son Ptolemy Philadelphus, the offspring of his fa- vorite wife Berenice, to succeed him, to the exclusion of his elder son by his former wife Eurydice, and effected his purpose by abdicating in his favor in 285, continuing however to exercise sovereignty until his death.