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.