People think of reading as the introvert’s hobby: A quiet activity for a person who likes quiet, save for the voices in their head. But in the 5,000 or so years humans have been writing, reading as we conceive it, an asocial solo activity with a book, is a relatively new form of leisure.

For centuries, Europeans who could read did so aloud. The ancient Greeks read their texts aloud. So did the monks of Europe’s dark ages. But by the 17th century, reading society in Europe had changed drastically. Text technologies, like moveable type, and the rise of vernacular writing helped usher in the practice we cherish today: taking in words without saying them aloud, letting them build a world in our heads.

Among scholars, there is a surprisingly fierce debate around when European society transitioned from mostly reading aloud to mostly reading silently—some even say the ancients read silently just as much as they read aloud—but there is one scene in literature they agree is crucial. In St. Augustine’s Confessions, the titular professor describes the reading habits of Ambrose, the bishop of Milan:

But when Ambrose used to read, his eyes were drawn through the pages, while his heart searched for its meaning; however, his voice and tongue were quiet. Often when we were present—for anyone could approach him and it was not his habit that visitors be announced to him—we saw him reading in this fashion, silently and never otherwise.

The fact that this was so remarkable to Augustine, some scholars argue, is because in the 400s, silent reading wasn’t really a thing.

Other researchers say that this passage is meant more to point out Ambrose’s rudeness. “It’s really that Ambrose would go on reading silently while he was there, like someone going on texting while you’re trying to talk to them,” says D. Vance Smith, a medievalist in the Princeton English department. “[Augustine is] surprised by his rudeness at not reading out loud to share with him.”

“The default assumption in the classic period, if you were reading around other people, you’d read aloud and share it,” says Smith. “For us, the default is we’ll read silently and keep it to ourselves.”

If silent reading was in fact rare or rude in ancient times, then at some point the expectation of readers in society shifted. As late as the 1700s, historian Robert Darnton writes, “For the common people in early modern Europe, reading was a social activity. It took place in workshops, barns, and taverns. It was almost always oral but not necessarily edifying.”

But by the time Marcel Proust was writing in the late 1800s, his narrator hoping for time to read and think alone in his bed, reading privately had become more of a norm for wealthy, educated people who could afford books and idle bedroom rumination.

This came with the spreading of literacy and diverse kinds of reading material. Writes Darnton, records from until as late as 1750 showed that people who could read had only a few books: perhaps the Bible, an almanac, and some devotionals, that they read and re-read. But by 1800, he writes, people were reading more voraciously—newspapers and periodicals—and by the late century they had branched out into children’s literature and novels.

As reading shifted away from the social, some researchers believe this helped create what we now call an interior life. Writes Alberto Manguel in his 1996 book, A History of Reading:

But with silent reading the reader was at last able to establish an unrestricted relationship with the book and the words. The words no longer needed to occupy the time required to pronounce them. They could exist in interior space, rushing on or barely begun, fully deciphered or only half-said, while the reader’s thoughts inspected them at leisure, drawing new notions from them, allowing comparisons from memory or from other books left open for simultaneous perusal. And the text itself, protected from outsiders by its covers, became the reader’s own possession, the reader’s intimate knowledge, whether in the busy scriptorium, the market-place or the home.

“Psychologically, silent reading emboldened the reader because it placed the source of his curiosity completely under personal control,” librarian Paul Saenger writes in his 1997 book, Space between Words. “In the still largely oral world of the ninth century, if one’s intellectual speculations were heretical, they were subject to peer correction and control at every moment, from their formulation and publication to their aural reception by the reader.” As Saenger writes, asocial reading helped facilitate intellectual rigor, introspection, criticism of the government and religion, even irony and cynicism that would have been awkward to read aloud.

This strange new trend of reading to oneself naturally had its detractors. Skeptics thought silent reading attracted day-dreamers and the “sin of idleness,” as Manguel writes. And worse: It let people learn and reflect without religious guidance or censure. Silent reading by the late 19th century was so popular that people worried that women in particular, reading alone in bed, were prone to sexy, dangerous thoughts.

There isn’t much consensus between historians on why people would have started reading silently. Saenger hypothesizes that a shift in the way words were laid out a page facilitated the change. Latin words once ran all together, makingithardtoparsethem. Saenger argues that Irish monks, translating Latin in the seventh century, added spaces between words to help them understand the language better. This key design change, he argues, facilitated the rise in silent reading.

M. B. Parkes, in his 1992 book, Pause and Effect: Punctuation in the West, argues something similar. He writes that a “grammar of legibility”—the visual changes made to texts, like punctuation and word spaces—changed the way we read. This early book technology was premised on the idea that the scribes, the people writing, didn’t know who their readers would be, or how fluent they might be in reading Latin, and so had to find a standardized way of telling them how to read: Pause here; these are two separate words; this is a long “a.”

This scholarship applies for the most part to the Latin-based writing and reading of Europe. In other major reading cultures of the world like Chinese, whose script doesn’t have spaces between words, and whose literature depends heavily on prosody, silent reading may have developed differently.

Mainstream historical accounts would have us think that the end of oral reading in the Middle Ages was part of the Renaissance, a new European preoccupation with the individual. But it’s possible humans’ desire for privacy, the carving out of a little pocket in which to escape by way of a book, was there all along. We just needed a little help getting there.