Much of the philosophical content of The Unbearable Lightness of Being begins with the idea of eternal return, or the notion that our lives are repeated ad infinitum throughout a circular passing of time. Kundera rejects the idea of eternal return and argues that our lives occur only once, and that time is in fact linear, not circular. Because our lives occur only once, they fail to gain weight or significance, and are unbearably light. Kundera compares human time, which is linear, to animal time and idyllic time in biblical Paradise, which he argues are circular. Kundera claims that happiness is the longing for repetition – the longing for circular time. We're out of luck, then, since we cannot experience time this way. Instead, we struggle to give our lives meaning and to be happy when neither are strictly possible.
The novel itself is the narrator's form of eternal return.
The bowler hat is a symbol of eternal return.