Until the introduction of alphabetic writing- "that recipe not for memory, but for reminding", as Plato says in the Phaedrus- the Greeks were forced to rely on the memorial powers of individuals, especially on those who had received special training. The mnemon, for example, was someone who kept track of proceedings in law courts without the benefit of written documents. In mythical representations, the mnemon was a servant of heroes who reminded them, at crucial moments, of divine injuctions. Thus Achilles was accompanied by a mnemon who was enjoined to warn him that if he were ever to kill a son of Apollo, he would be put to death.

Casey E., Remembering, a phenomenological stydy, p.11

Lesmosyne derives from the same root as Lethe and means exactly the same thing [ i.e., forgetfulness]. The sphere of the Muses, which arises from the primordial Goddess Mnemosyne, also has the benefit of Lethe, who makes everything disappear, that belongs to the dark side of human existence. It is only both the elements-giving illumination and letting disappear, Mnemosyne and her counter-pole, Lesmosyne-that make up the entire being of the Goddess, whose name comes solely from the positive side of her field of power. This is a union of the opposites under the dominion of the positive.

Karl Kerenyi, "Mnemosyne-Lesmosyne. On the springs of the Memory and Forgetting", in Casey Edward, Remembering, a phenomenological study, p.12