In preparation to leave Hades for its next incarnation, a dead soul would as a matter of course take a draught from the River Lethe, the River of Forgetfulness or Oblivion, in order to forget its past life and begin anew. Ancient inscriptions exhort the adherents of one mystery tradition to drink instead from the River Mnemosyne, the River of Memory, and so, over time, become all-knowing as lifetimes of information built upon themselves. The goddess Mnemosyne, one of the Titans (Old Ones) given life by Heaven and Earth, is widely revered as the Mother of the Nine Muses by her nephew, Zeus; and, though few tales remain, she is known to give humankind all things relating to the function of memory, including language and inspiration. Most notably, She gives us a sense of understanding of the world around us, allowing interactions and encounters to stick in our minds and so build images of ourselves in relation to the universe, now and throughout the ages. All of our existence -- past, present and future is rooted in Her.
is the best and most complete picture I've found thus far)
"To Mnemosyne (Memory), Fumigation from Frankincense. The consort I invoke of Zeus divine; source of the holy, sweetly speaking Mousai nine; free from the oblivion of the fallen mind, by whom the soul with intellect is joined. Reasons increase and thought to thee belong, all-powerful, pleasant, vigilant, and strong. Tis thine to waken from lethargic rest all thoughts deposited within the breast; and nought neglecting, vigorous to excite the mental eye from dark oblivions night. Come, blessed power, thy mystics memory wake to holy rites, and Lethes (Forgetfulness) fetters break." - Orphic Hymn 77 to Mnemosyne
(Thanks to Jenniforensic for the info and the invocation.)