"What does change mean?" And how does one define the delicate limit between remaining oneself and becoming another? Through this preliminary question, Etienne Klein examines the concept of identity and invariability that applies from the smallest elementary particle to the infinite universe, but also to each human person. With clarity and simplicity, he leads a reflection at the crossroads of philosophy, physics, and politics. In the form of a lecture followed by an exchange with the public, Etienne Klein also returns, in a fascinating and educational way, to the questions and the scientific theories of the origin of the universe, and the biggest questions of physics.