Mnemosyne (Greek: Mνημοσύνη), source of the word mnemonic, was the personification of memory in Greek mythology. A Titanide, or Titaness, she was the daughter of Uranus and Gaia, and the mother of the nine Muses by Zeus:
In Hesiod’s Theogony, kings and poets receive their powers of authoritative speech from their possession of Mnemosyne and their special relationship with the Muses.
Zeus and Mnemosyne slept together for nine consecutive nights, thus birthing the nine Muses. Mnemosyne also presided over a pool in Hades, counterpart to the river Lethe, according to a series of 4th century BC Greek funerary inscriptions in dactylic hexameter. Dead souls drank from Lethe so they would not remember their past lives when reincarnated. Initiates were encouraged to drink from the river Mnemosyne when they died, instead of Lethe. These inscriptions may have been connected with Orphic poetry.
Similarly, those who wished to consult the oracle of Trophonius in Boeotia were made to drink alternately from two springs called “Lethe” and “Mnemosyne”. An analogous setup is described in the Myth of Er at the end of Plato’s Republic. Source Wikipedia.