Strength: Franz laughs at his own physical strength and considers it useless in his city life. He believes love "means renouncing strength." Sabina admires his strength and wishes it translated internally, or that he would use his strength on her.

Living in truth: Franz views privacy, including secrecy in love, as a form of deception. In order to feel that he lives in truth, he decides to leave his wife and live with Sabina. For Sabina, on the other hand, living in truth means living privately, not allowing the public to see or interfere in one's life. A secret love is the only kind she could conceive of.

Franz's wife Marie-Claude holds a dinner party for local artists. Her loud, vulgar hostessing offends Franz. Sabina enters and Marie-Claude accosts her, commenting loudly on the ugliness of Sabina's pendant. (She says this to put the beautiful Sabina in her place, not because she suspects that Franz and Sabina are having an affair).

Franz thinks of his admiration for the simple Amsterdam church, and decides to sweep his life clean. He packs his bags and tells Marie-Claude he has been having an affair. She reacts neither with pain nor surprise, but with quiet aggression. Franz leaves to meet Sabina.

Sabina tries to convince herself she must put an end to her lifelong pattern of betrayal, but she feels increasing distaste for Franz. That night when they make love and Franz again closes his eyes, she thinks of him as a grotesque oversized newborn. Both are drunk with joy, Franz because he thinks he will spend the rest of his life with Sabina, Sabina because she has resolved to leave Franz the next day.

Franz finds out Sabina has left and understands nothing. He finds a small apartment and starts living alone for the first time in his life. Preferring the ideal to the real, he finds happiness imagining Sabina as a guiding presence in his life. He starts living with one of his students, a kind young girl with thick glasses. Franz's wife refuses to grant him a divorce, but he lives happily with his new mistress, sharing his ideals and dragging her to Czech dissident émigré conferences.

Sabina moves to Paris. She wonders where to go next, and if her betrayals will lead to death. Tomas's son informs her by letter that Tomas and Tereza have died. Sabina thinks of death. To her no more terrifying thought exists than the image of a heavy stone trapping her underground.

Analysis

Franz and Sabina represent such extremes of heaviness and lightness that they fail to understand each other. Unlike Tereza and Tomas, who torture each other but more or less understand how the other feels, Sabina and Franz interact like members of a different species.

Sabina is so light that she betrays again and again. While she is tired of running away and would like to meet someone strong enough to hold her down, and while she understands that Franz is the best man she has ever known, Sabina reacts to heaviness with instinctive distaste. And Franz, the man she almost loves, is supremely heavy. As Tereza does, Franz imbues everything in his life with profound meaning. Even Tereza doubts that meaning sometimes, but Franz does not entertain doubts.

The unbearable lightness of being excites Sabina, and she is excited by the thought of betraying such a good loving man for no meaningful reason.

Even though characters and human beings can follow only one path, authors have more flexibility; Kundera, for example, can experiment and see what happens when different choices are made. This seems to be Kundera's (serious) joke: he takes two couples, both composed of one light and one heavy person, and makes them do different things. Thus he gives the reader what the characters complain they lack: the ability to compare paths. Tomas and Tereza decide to stay together; Sabina and Franz part ways.

Kundera also uses Franz to level a devastating and cynical critique against the European left. Leftist beliefs are painted as the ultimate naive idealism on the part of scholars who have spent their lives indoors and understand nothing of revolution's horrors.