This book can be used as a textbook for a graduate or advanced undergraduate course. The material could be covered in two semesters or two quarters. An earlier version has been used as a textbook at the Catholic University of Chile by the author and other teachers for several years. The main requirements for the study of this book are mathematical maturity and some knowledge of Elementary Logic. This knowledge can be provided, for instance, by Enderton 1972 Chapters 1-2. Acquaintance with set theory at least on the level of Halmos 1965 or Enderton 1977 is also necessary although not strictly required from a formal point of view. The formal style of this book would make it difficult to understand without previous acquaintance with the intuitive notions.