2 With reference to these objections, Plato, that ancient Athenian, who also was the first to introduce this opinion, when he could not set them aside, invented the [notion of] a cup of oblivion, imagining that in this way he would escape this sort of difficulty. He attempted no kind of proof [of his supposition], but simply replied dogmatically [to the objection in question], that when souls enter into this life, they are caused to drink of oblivion by that demon who watches their entrance [into the world], before they effect an entrance into the bodies [assigned them].
It escaped him, that [by speaking thus] he fell into another greater perplexity. For if the cup of oblivion, after it has been drunk, can obliterate the memory of all the deeds that have been done, how, O Plato, do you obtain the knowledge of this fact (since your soul is now in the body), that, before it entered into the body, it was made to drink by the demon a drug which caused oblivion? For if you have a remembrance of the demon, and the cup, and the entrance [into life], you ought also to be acquainted with other things; but if, on the other hand, you are ignorant of them, then there is no truth in the story of the demon, nor in the cup of oblivion prepared with art.
Source: Adversus haereses (New Advent)