Lightness and weight both get linked to a philosopher, a philosophy of life, and several characters. The ancient Greek Paremenides, mentioned in the opening pages of the novel, is a philosopher of lightness to whom weight is negative. Practically, accepting the lightness of being means accepting a certain lack of ultimate meaning in life, and living for momentary beauty. Those who accept lightness, for example, are not likely to ally themselves to political parties, either the Communist regime or the dieh ard dissidents. While both Tomas and Sabina are characterized by lightness, Sabina is the more extreme example as she consistently refuses to be tied down. Tomas, on the other hand, ultimately returns to Tereza and Prague.
Kundera associates heaviness with Nietzsche and the philosophy of eternal return. Kundera does not believe eternal return exists, and argues that man only has the opportunity to try one path, and hence has no point of comparison or meaning. Instead, those characters who are heavy cannot accept this unbearable lightness of being, and seek to attach a meaning and weight to what they consider important in life. Tereza and Franz are both heavy characters. Tereza is heavy emotionally and cannot cope with the lightness around her, and is driven nearly to insanity. Franz, interpreting all the events of his life as heavy, is led to an early and unnecessary death.
Lightness versus weight is the key dichotomy of The Unbearable Lightness of Being, a paradox that cannot be resolved. None of the four characters ultimately seem to find a solution. It is noteworthy that of the four, Sabina is the only one living at the end of the book; however, not even she is necessarily happy or fulfilled or sure of her life choices.
In The Unbearable Lightness of Being, politics exist as background, and in simple terms. The three more perceptive characters in the novel, Sabina, Tomas and Tereza, all recognize or come to recognize one of Kundera's most important ideas, that all diehard political or ideological parties are fundamentally the same. Sabina recognizes this on an artistic level, seeing that Communists, Fascists and the extremely religious all employ sentimentalist kitsch, bad art, and propaganda. Tomas realizes this when both the Czech regime and dissident groups pursue him and want him to sign something; he realizes that both groups plan to use and misrepresent his words. Tereza, while originally tempted by dissident glamour, understands on an intuitive level that all political parties are anti-individualistic and would deny her privacy. None of these three characters, by the end of the novel, would be comfortable participating in a march or rally, and hence all could be identified as apolitical.
On the other hand, Kundera's characters hold strong personal beliefs that have political implications. Tomas's Oedipus article says ignorance of the law is no excuse, and damns the Communist regime because it uses ignorance to explain away its wrongdoing. Sabina requires freedom to pursue experimental painting, and finds that freedom in the West, away from the Communists in Prague.
Ultimately, Kundera portrays the Soviet military occupation of Prague as horrifying. The foreign tanks and soldiers present a rape of the beautiful city, and the regime damns its most intelligent men and women to exile or permanent silence. The cultural world to which Tomas and Sabina belonged in the early days of the novel has been permanently destroyed.
Kundera presents sexuality in terms of lightness and weight. The lighter characters of the novel are strongly erotic and view their sexuality as generative and creative. Sabina paints as imaginatively as she makes love; Tomas heals individual patients wit h the same precision that he uses to seduce individual women. Neither feels any degree of guilt for their sexual promiscuity. The heavy characters of Tereza and Franz, however, are marked by sexual guilt. Tereza hates bodies, especially her own, and Tomas 's extramarital affairs destroy her. Her one affair with the tall engineer is a disaster that almost drives her mad. Franz too is tormented, specifically by the thought that he must betray his wife for Sabina. He cannot bear to move from one woman's bed to the other, and ultimately leaves his wife to avoid deceiving her. He also refuses to use his physical strength in bed, stating that "love means renouncing strength."
Kundera also presents sexuality in gendered terms. Feminist critiques of Kundera have drawn attention to this fact, and level the accusation that in all of his novels, especially in The Unbearable Lightness of Being, sex is equated with rape or violence. Whether one considers the accusation valid or irrelevant in the context of the novel, the sexual comfort and joy the characters feel seems directly related to their acceptance of certain gendered behaviors. Unlike Franz, Tomas is sexually successful because he is not afraid to use his strength (a stereotypically male trait), but instead issues the infamous command, "Take off your clothes!" to women who almost always obey. Franz, who does not use his strength, is portrayed as weak, and a sometimes unsuccessful lover. Sabina, unlike Tereza, enjoys relinquishing power (a stereotypically female way of behaving). She thinks of the bowler hat as provocation, a "submitting of her own will to public rape." She enjoys the role of the stereotypically weaker woman, so she enjoys sex more than does Tereza. Tereza, contemplating her submission to the tall engineer, feels she must leave Prague or die of shame.
The body itself presents a separate paradox, as it cannot be clearly identified as heavy or light. The philosopher Parmenides would classify the body as heavy and the soul as light; however in a contemporary setting, those that choose the soul over the body seem the heavier. Of the four main characters, Franz and Tereza are the only ones who would even use a word like "soul." They search for a higher, nobler meaning of individuality than what they see in the body. Franz, after all, is a scholar and professor, and Tereza a political photographer. Both are interested in lofty ideas and convictions. Sabina and Tomas, on the other hand, are firmly rooted in and attached to the body. Sabina is a visceral painter, Tomas a doctor; both have many lovers and are fascinated by the human body.
In terms of sexuality, Sabina and Tomas are comfortable with their bodies and interested in those of others. Franz, on the other hand, considers his strong muscular body useless in a civilized world, and refuses to use his strength in a sexual context. Tereza goes further. She is revolted by the naked body, considering nakedness and sexuality potential horrors.
The body also figures as a symbol of death, as a corpse. Ultimately none of the characters are prepared for or have an understanding of death. Across all divisions of lightness or weight in philosophy, all the characters have reason to fear the dead body. The abandoned wife appropriates Franz's corpse, and the abandoned son appropriates Tomas's body. Tereza, for all her revulsion of the body, cannot escape becoming a corpse. Sabina, the only character left living, understands that her death will mean an end to all wanderings, and shudders at the thought of a heavy stone placed over her body.
The bowler hat, which Sabina describes as a leitmotif in her life, acts in the novel as a symbol for her eroticism and rebellion. The hat, which originally belonged to her grandfather, has been made (originally out of mischief) into an erotic plaything by Sabina and Tomas. When Franz removes the bowler from her head before making love, it symbolizes the stifling effect he has on what Sabina views as her most important characteristics, her unabashed eroticism and betrayal of roots.
Tereza's suitcase, into which she packs all her belongings before coming to Prague, represents her whole life and philosophy. Tomas understands this instantly when she arrives at his door; he knows she has come "to offer him up her life," and that her life is in that suitcase. The heaviness of the suitcase also represents Tereza herself, for as Tomas carries the heavy object into his house, he voluntarily accepts the burden their love will bring him, and the heaviness of Tereza's character.
Tereza falls instantly in love with Tomas because he has a book on his café table. Growing up with a vulgar mother and in a town where no one reads, Tereza sees the book as a symbol of a secret brotherhood by which she can recognize Tomas as hers. The secret brotherhood of the book remains their point of connection, as they name their dog, their only mutual possession and responsibility, after a character in the book Tereza was reading the first day.