Lethe and Mnemosyne

In Greek Mythology Lethe and Mnemosyne meaning forgettfullness and Memory were two concepts that were personified in different forms.

Mnemosyne 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.
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 and thereby created the nine muses.

Mnemosyne was also the name for a river 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. Those who drank from the Mnemosyne would remember everything and attain omniscience. Initiates in the Orphic and Eleusinian Mysteries were taught that they would receive a choice of rivers to drink from after death, and to drink from Mnemosyne instead of Lethe. These two rivers are attested in several verse inscriptions on gold plates dating to the 4th century BC and onward, found at Thurii in Southern Italy and elsewhere throughout the Greek world. Those plates gave instructions to the dead. When he comes to Hades, he must take care not to drink of Lethe ("Forgetfulness"), but of the pool of Mnemosyne ("Memory"), and he must say to the guards:
"I am the son of Earth and Starry Heaven. I am thirsty, please give me something to drink from the fountain of Mnemosyne."