Syntactic categories constitute the core of grammatical theory, and this book examines their ontological structure across four levels of abstraction: individual categories, functional sequences, structured sets of sequences, and the comprehensive space of all possible functional-sequence sets. While prior studies typically focus on the second level, this work uncovers the intricate structures emerging at higher levels of abstraction. Central to its approach is the interplay between category-generating templates and some formal flexibility at each level, which together account for the diversity of syntactic categories. By exploring these template–flexibility pairs, the book integrates key topics such as functional hierarchies, root syntax, and category subtyping, advocating for a category-first perspective on syntax. Combining deductive reasoning with case studies that draw on data from diverse languages, it employs mathematical category theory to model the syntactic category system. This book is a valuable resource for linguists and researchers in syntax, theoretical linguistics, formal grammar, and typology. It also appeals to cognitive scientists and mathematical linguists exploring the structural foundations of language and cross-linguistic variation.
