The subject of this book emerged from a series of lectures that the author gave at the Department of Physics of the University of North Texas during the 1992 Spring Semester, and reflects the vivacious discussions that he has been having with the students and the co-workers attending this course.

The main conclusion of these discussions was that the major tenet of the "conservative" physicists, that classical physics must be recovered from quantum mechanics by adopting the statistical perspective of Gibbs, implying by necessity a Gibbs ensemble of Universes as well as a Gibbs ensemble of observers, is not satisfactory. It is actually as unsatisfactory as the dominant approaches to irreversibility. The book examines the current approaches to irreversibility, in classical and quantum physics, and shows that an objective theory of irreversibility does not exist yet, and that all the current theories of irreversibility share with quantum mechanics elements of subjectivity, making crucial the role played by the observer. In addition to the traditional quantum mechanical paradoxes, concerning the quantum theory of measurement, the book also discusses the new difficulties that the physics of chaos is causing to the widely accepted correspondence principle, and suggests that the Boltzmann dream, the dream that the fracture between dynamics and thermodynamics might be healed, cannot become true within the framework of the current physics, and that the establishment of a new physics is necessary for that ambitious purpose to be achieved.