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 laid with Mnemosyne for nine consecutive nights and thereby created the nine muses. Mnemosyne was also the name for a river in Hades, counterpart to 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 a private mystery religion, or with Orphic poetry (see Zuntz, 1971).
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.
Asteroid 57 Mnemosyne
Zuntz, Gunter. Persephone. Cambridge, 1971.