This book is an introduction to law. This means that, for the most part, it is directed both to people at the beginning of their study of law (be it in a faculty of Law or in a faculty of Canon Law) and to those who are already experienced jurists or canonists who wish to recall or review the foundations of their craft. In a certain sense, then, this is a rudimentary book, but it is far from being a publication for the general public. For this reason I have intended to be clear, but I am not guaranteeing that it will always be easy reading. Moreover, I dare say that this is no practical introduction. It can be classified as original in some ways. It is an introduction to law from the perspective of classical juridical realism (understanding law [derecho] as what is just). Although it is a perspective as old as the Roman jurists, at the beginning of the 14th century it was practically replaced by subjectivism (understanding law as a subjective right), and then by normativism (understanding law as a norm), which is still the dominant perspective today. For this reason, returning to juridical realism is an attempt at renewing and modernising the juridical science. It is not a view toward the past, but a purification of the science of law from a frail and antiquated vision, which has sufficiently demonstrated its own sterility and the deforming effect it has had on the work of the jurist. In this sense, this book can be considered a synthetic and propedeutic exposition of classical juridical realism—a method that is different from the usual way of learning law. For this reason I believe this book can be of interest to jurists and canonists alike. Of everything that is said in this introductory book, I think it most important that this idea be well illustrated: the final, fundamental goal of the science of law is that a society may be just, with that real and concrete justice that consists of respecting and giving to each person the rights that are his or hers. This task is important and of incalculable social transcendence, even though achieving it may at times require great efforts. It is a task in which it is entirely worthwhile to persist.