recursive, as they may consist of other constituents, potentially of the same type. Early history The Aṣṭādhyāyī of Pāṇini, from c. 4th century BC in Ancient India, is often cited as an example of a premodern work that approaches the sophistication of a modern syntactic theory since works on grammar had been written long before modern syntax came about. In the West, the school of thought that came to be known as "traditional grammar" began with the work of Dionysius Thrax. For centuries, a framework known as , first expounded in 1660 by Antoine Arnauld in a book of the same title, dominated work in syntax: as its basic premise the assumption that language is a direct reflection of thought processes and so there is a single most natural way to express a thought. However, in the 19th century, with the development of historical-comparative linguistics, linguists began to realize the sheer diversity of human language and to question fundamental assumptions about the relationship between language and logic. It became apparent that there was no such thing as the most natural way to express a thought and so logic could no longer be relied upon as a basis for studying the structure of language. The Port-Royal grammar modeled the study of syntax upon that of logic. (Indeed, large parts of Port-Royal Logic were copied or adapted from the Grammaire générale.) Syntactic categories were identified with logical ones, and all sentences were analyzed in terms of "subject – copula – predicate". Initially, that view was adopted even by the early comparative linguists such as Franz Bopp. The central role of syntax within theoretical linguistics became clear only in the 20th century, which could reasonably be called the "century of syntactic theory" as far as linguistics is concerned. (For a detailed and critical survey of the history of syntax in the last two centuries, see the monumental work by Giorgio Graffi (2001).) Theories There are a number of theoretical approaches to the discipline of syntax. One school of thought, founded in the works of Derek Bickerton, sees syntax as a branch of biology, since it conceives of syntax as the study of linguistic knowledge as embodied in the human mind. Other linguists (e.g., Gerald Gazdar) take a more Platonistic view since they regard syntax to be the study of an abstract formal system. Yet others (e.g., Joseph Greenberg) consider syntax a taxonomical device