The waters of Lethe remove all memory, all guilt, all shame, and all knowledge. In Greek mythology, it allowed a soul to enter into Hades with their guilt expunged. Or return, again. In Chinese mythology, the Goddess of Mercy gives a tea of forgetfulness before the soul is reincarnated: there is a thought, however, that the soul will learn. Over turns at the cycle.
Donne knows that there is no reincarnation, and those animals without moral agency cannot be damned. He knows that we should not return to our instincts, and be brute beasts. And he knows that God accounts our sins as nothing only because of the death of Christ, now risen.
If poisonous minerals, and if that tree,
Whose fruit threw death on (else immortal) us,
If lecherous goats, if serpents envious
Cannot be damn'd, alas ! why should I be?
Why should intent or reason, born in me,
Make sins, else equal, in me more heinous?
And, mercy being easy, and glorious
To God, in His stern wrath why threatens He?
But who am I, that dare dispute with Thee?
O God, O! of Thine only worthy blood,
And my tears, make a heavenly Lethean flood,
And drown in it my sin's black memory.
That Thou remember them, some claim as debt;
I think it mercy if Thou wilt forget.
John Donne, 1633