Mnemosyne was born among the twelve Titans, the vast first generation of gods sired by Uranus, the starry Sky, upon Gaia, the broad-breasted Earth. Her brothers were Cronus the crooked-minded, who would one day castrate his father and devour his own children, and Oceanus, the river that rings the world; her sisters were Rhea, the mother of Zeus, Tethys, the queen of the waters, and Themis, the power of divine law. Mnemosyne's gift was quieter and stranger than any of theirs: she was Memory, the mind's power to hold the past, given the shape of a goddess. When Cronus rose against Uranus and struck him down with a flint sickle, and years later when Zeus waged the Titanomachy, the ten-year war between the old Titans and the young Olympian gods, Mnemosyne took no part in the fighting. The poets never put a spear in her hand. Her weapon was remembrance, and it proved stronger than any spear, because the gods themselves would need her before long.

After Zeus had won his throne and divided the spoils of the cosmos, he came to Mnemosyne in the hills of Pieria, a green district at the foot of Mount Olympus, the gods' high mountain home. For nine nights the king of the gods lay with her, far from the eyes of his jealous queen, and in time Mnemosyne bore nine daughters, one for every night of love. These were the Muses, the goddesses of every art the Greeks held sacred: Calliope of the epic song, the eldest, whose voice Homer himself would borrow to tell of Troy; Clio of history, with her open scroll; Euterpe of music, with her double flute; Thalia of comedy, Melpomene of tragedy, Terpsichore of the dance, Erato of love poetry, Polyhymnia of the sacred hymn, and Urania of the stars, with her globe and rod. The Muses made their homes on Mount Helicon in Boeotia, where the spring Hippocrene, the Horse's Fountain, burst from the rock at the stamp of the winged horse Pegasus, and on the slopes of Olympus, where they sang at the feasts of the gods. The poet Hesiod, a shepherd of Helicon who claimed the Muses themselves had breathed their song into him, called the nine sisters the forgetfulness of ills and a rest from sorrows: it is Memory's daughters, and only Memory's daughters, who can make a grieving man forget his grief for a while.

Mnemosyne's strangest realm lay below the earth. In the secret rites called the Orphic mysteries, named for Orpheus, the legendary singer who had descended alive into the underworld to fetch back his dead bride, the afterlife was imagined as a crossroads with two springs. On one side flowed Lethe, Forgetfulness, and the uninitiated dead drank from it and lost themselves; on the other side, beside a white cypress tree, rose the cold spring of Memory. From about 400 BCE, initiates in southern Italy, Crete and Thessaly were buried with thin gold tablets inscribed with directions for the dead, tiny passports folded into the grave. The tablets tell the soul to go to the right, past the white cypress, past the spring of Forgetfulness, to the cold water flowing from the Lake of Memory, and there to say to its guardians: I am a child of Earth and starry Heaven, I am parched with thirst and perishing, give me quickly the cold water of Memory. The guardians then give the soul to drink, and it passes on to the seats of the blessed. For these worshippers, salvation itself was an act of remembering, and Mnemosyne held the keys to paradise.

Memory had one more shrine, and it worked like a machine. At the oracle of Trophonius in the Boeotian town of Lebadeia, where the hero Trophonius spoke prophecies from a cave beneath the earth, every visitor was made to drink from two springs before descending: first the water of Lethe, to empty the mind of every previous thought, and then the water of Mnemosyne, so that whatever he saw in the darkness below would be fixed in his memory forever. On Mount Helicon the oldest worship of the Muses honoured just three sisters, and one of them was Mneme, Memory herself, alongside Melete, Practice, and Aoide, Song: to the earliest Greeks, practice, memory and song were the whole of art. Centuries later the poet Simonides of Ceos, as the Romans told it, was called out of a banquet hall moments before its roof collapsed and crushed every guest beyond recognition, and he identified each body for its family by remembering exactly where each man had been sitting. From this the Greeks and Romans built the art of memory, the training of the mind that their orators used to hold whole speeches without a single note. A Titaness had quietly become a science.