From its very inception, quantum mechanics troubled physicists. It seemed to challenge our conception of reality and lead to apparent contradictions. One of the founders of quantum mechanics, Werner Heisenberg, questioned whether the theory offered a description of reality at all. Others, like Niels Bohr, claimed that somehow human consciousness played a role in the theory. In this interview, Carlo Rovelli explains Heisenberg’s anti-realist motivations, clarifies the role of the “observer” in quantum mechanics, and articulates his relational interpretation of the theory, according to which reality is a network of interactions.