This is a book about the nature of the reality or the phenomenon referred to as “law” by lawyers and legal officials, but also by members of communities living under law, to the extent that they conceptualize the realm of “law” as something sufficiently distinguishable from other different or broader realities and phenomena.

The book belongs to the legal-philosophical field dedicated to the study of the very nature of law, that is, to the most basic or abstract part of the philosophy of law, which is sometimes referred to as “the philosophy of philosophy of law” or “general jurisprudence.”

The scope of the book is to explore the essential features of law’s nature, namely, all those realities that pertain to law’s essence, the properties that make law into what it is. Despite the fact of plurality of legal-philosophical conceptions of law circulating in legal theory today, the book is optimistic about the very prospect of exploring the nature of law. It maintains that law has an essence and that it is possible to explore and describe this unique essence in an intelligible and reasonably detailed fashion, by identifying the basic elements of law’s ontology.

The book advances the claim that Aquinas’s account of legal ontology denotes the most accurate understanding of the phenomenon that we call “law”—which the Angelic Doctor referred to as ius, constituted as such on the basis of the various manifestations of lex. Aquinas’s approach to understanding law’s nature that can be labeled “Thomistic juridical realism” and it may be categorized as a version of a natural-law theory of law. The book argues that a contemporary reading of Aquinas may indeed fruitfully contribute to an improved understanding of our present-day legal-philosophical analyses and discussions on the nature of law.