Piccarda Donati is the first soul Dante speaks with in Paradise, in canto III, in the heaven of the Moon, the lowest. She was a nun, she followed the rule of a higher woman in heaven, whom tradition reads as Saint Clare, and men more inclined to evil than to good dragged her out of the sweet cloister. That is why she is among the souls who did not keep a vow.
She explains why she is there: their vows were neglected and left empty, and the poem plays on the words "vows" and "empty." Dante asks whether they do not desire a higher place, and Piccarda answers that their will is calmed by charity, which makes them want only what they have. If they wanted to be higher, their will would disagree with the will of the one who placed them there. The sentence that closes the argument is this: in his will is our peace.
Her companion is Constance, whom Piccarda points out to the right. Then she sings "Ave Maria" and vanishes singing, like a heavy thing sinking in deep water. The scene fixes a rule of Paradise: the rung does not measure the happiness of the one who occupies it, it measures the way grace arrives. In canto IV Beatrice distinguishes absolute will, which does not consent to harm, from the will that consents out of fear of a greater evil.