The central question of the novel comes from Nietzsche’s words on eternal return:

“What then shall we choose? Weight or lightness?”

This question is not rhetorical, it frames the entire novel’s metaphysical inquiry. 

Eternal Return:

Nietzsche's concept of eternal return is a thought experiment, not a cosmological claim. Imagine that you must live your life over and over again, infinitely, with every detail repeated exactly. Every joy, pain, embarrassment, every trivial and profound moment is lived out. The perpetuity of a life led Nietzsche to ask the question:

Would you rejoice at the eternal return of your life, or would it be a curse?

This is not about reincarnation or fate. It is a radical ethical test: do you live in such a way that you would embrace repeating your life forever? If you choose that your life is immensely weighted, every single, minute choice made matters infinitely. If your life has no weight, you are completely disconnected from meaning.

Kundera quotes “Einmal ist keinmal,” — a German saying, meaning that if something happens only once, it may as well have never happened. If eternal return does not exist, everything becomes light (or insignificant). Unlike Nietzsche, Kundera does not embrace weight as a moral necessity. He mourns the lightness of our existence in a world where actions have no eternal echo, and thus, no ultimate meaning.

This dilemma is embodied through Tomas, the novel’s protagonist. He seeks lightness and lack of burden in erotical freedom and his anti-ideological stance. He desires a life free of responsibility and repetition. But as the novel progresses, his life is burdened by meaning. His love for Tereza (despite his constant infidelity) and his guilt, add weight to his life.

Lightness versus Heaviness:

While lightness represents freedom from obligation, and detachment while heaviness is a sense of responsibility and moral burden, it does not cast a sharp divide of good or bad. Lightness while liberating, can also be alienating. A life of detachment becomes shallow, unmoored. Heaviness, while painful, can also bring depth, meaning, and beauty.

Kundera never resolves the tension between these forces. Instead, he explores their interplay. He deems it that we find meaning as we oscillate between wanting lightness and needing heaviness. Instead of offering a  solution to this dilemma, Kundera asks readers to live with the knowledge that life is light, and yet we must carry it as if it were heavy. The ethical demand: is not to flee from lightness into ideology, nor to float into utter detachment, but to live in between. To be, in this world, is to dance between these opposites. We must bear lightness as if it were heavy, and to bear heaviness lightly enough to remain human.

Sources:

https://www.newyorker.com/magazine/2019/10/14/nietzsches-eternal-return

https://scholarcommons.scu.edu/phi/55/