The term “index” didn’t come to be used in its current sense in English until quite recently. There’s no entry for it in Cawdrey, while Samuel Johnson’s Dictionary of the English Language (1755) defined it as “the table of contents to a book.” For centuries words like “table,” “register,” and “rubric” were used interchangeably for what we now call indexes and contents pages: only very gradually, over the past two hundred years or so, have the two forms come to be regarded as essentially distinct. (The related Anglophone convention that indexes appear at the back of books and contents pages at the front seems to have been part of a similar, relatively recent process of separating them.) Nonetheless, names aside, the alphabetical index as a type of textual technology turns out to be much older than that.
There are two basic types, which in modern books are usually combined into a single list. One type collates words, the other concepts. The former is a concordance, the latter a subject index. The first is the kind of literal, specific listing of entries that you can get your computer to produce by using CTRL+F for any word or phrase in a text; the second is the product of a more subjective, humanistic attempt to capture the meanings and resonances of a work. Duncan, understandably, is mainly interested in the evolution of the latter type—even though, because of the rise of the word-based online search engine, we now find ourselves living in a golden age of the concordance. But the two forms need to be treated together, he argues, because both were invented at the same time and place: in northwestern Europe, in or around 1230.
The index was, in fact, part of an entire range of organizational and reading tools that were conceived in the thirteenth century. (Others included the division of the Bible into standard chapters and the genre of distinctiones, a new kind of search aid for preachers that helpfully grouped together biblical extracts on the same subject.) Two social developments at this time created a novel demand for information to be organized in rapidly accessible ways: the foundation of the first European universities and the rise of the new orders of mendicant friars, with their stress on the frequent and engaging preaching of God’s word. In Oxford the scholar and cleric Robert Grosseteste (so named, it was said, because of his gigantic brain) compiled an enormous subject index to the whole of the world’s knowledge, to aid him and his students in navigating it all. His Tabula, of which only a fragment survives, encompassed the entirety of the Bible, the writings of the Church Fathers, the works of classical authors like Aristotle and Ptolemy, and Islamic authorities including Avicenna and al-Ghazālī.