From Wikipedia:
Mnemosyne (pronounced ni'mazini in RP i.e. standard English) was the personification of memory in Greek mythology. This titaness was the daughter of Gaia and Uranus and the mother of the Muses by Zeus.
Zeus laid with Mnemosyne for nine consecutive nights and thereby created the nine muses.
And finally: Mnemosyne was also the name for a river in Hades, counterpart to Lethe, Dead souls drank from Lethe so they would not remember their past lives when reincarnated. Initiates of Orpheus were encouraged to drink from the river Mnemosyne when they died, instead of Lethe.