This book is a remarkable synthesis, a clear and simple introduction to Quantum Physics with a sort of Galilean dialogue on the supreme systems of contemporary Physics. The author, whose research interests and work extended from quarks to liquid systems and from crystals to stars, introduces the common conceptual and mathematical framework of all quantum theories, realistic enough to successfully confront Nature: Quantum Field Theory applied to the study of both dilute and condensed matter. In the dilute limit, quantum mechanics is shown to be a good approximation to Quantum Field Theory. However, in condensed matter physics the problem of the ground state, which acts as a kind of template for physical reality, is studied under the hypothesis that the standard perturbative vacuum is unstable with respect to a new coherent vacuum, whose spectrum emerges quite naturally through a simple variational procedure.