Modou claims that he's only marrying Nadou out of duty. He's marrying her to satisfy his mother, who claims that she will die of shame if her son doesn't accept the "gift" given to him by Farba, Modou's maternal uncle.

Yet Aissatou, Modou's current wife, is thoroughly unconvinced of this line of reasoning. In her goodbye letter, she expresses her profound opposition to society's belief that princes should master their feelings in carrying out their princely duties. Modou claims to be able to separate heartfelt love and physical love. He still retains the former for Aissatou, and will enjoy the latter with his new wife Nadou. But Aissatou rejects what she sees as this false distinction. As far as she's concerned, there can be no physical union between bodies without the heart's acceptance of the bond.

Aissatou leaves her goodbye note on the bed she used to share with Modou. This is a highly symbolic act, as Aissatou has effectively been banished from the marital bed since Modou took another wife. Her leaving the letter on the bed is a way of challenging Modou's decision, of showing that she belongs in the marital bed. And as she can no longer do this physically, she'll do it symbolically by way of a letter.

Modou seems to think that he can separate himself into two different men: one man devoted to Aissatou as a loving husband, the other sleeping with Nabou out of a sense of duty. But Aissatou doesn't accept this. In her letter, she tells Modou that man is one entity, both greatness and animal fused together. Contrary to what Modou seems to think, no action is pure charity or pure bestiality. In other words, his "selfless" decision to marry Nadou is bound up with less savory motives, such as his animalistic need to satisfy the desires of the flesh.