How do you turn an unthinkable idea into one that everyone can accept, one that becomes a cultural commonplace? Not quickly, is one answer. Another answer is by squeezing it through the Overton windows.
The Overton window is a concept named after Joseph Overton which tries to explain the political acceptability of a policy at any given time, as it moves across a number of degrees of acceptance ranging from unthinkable (farthest removed) to policy (accepted, embraced and made practice). Although the Overton window is usually applied to political ideas, it is relevant for a broad range of cultural ideas, of which political ones are a subset. So a new idea like same-sex marriage reaches the public and political discourse as an unthinkable idea first. As the larger spectrum moves towards freedom and civil rights, the idea becomes merely radical. Through subsequent steps, it is rendered acceptable by the public, then it is seen as sensible, popular and finally becomes policy. This admittedly simplistic schema may appear linear, but in reality things are never so. Nevertheless, the Overton window is a useful approach to understanding that every idea in every age has sat in such a window, gazing at the shifting levels of acceptability in the public domain.
When we turn to premodern societies (or any society for that matter), we notice that the Overton window was still open. In the West, the translation of the Bible into the local language was unthinkable before, say, the 1300s. As the spectrum of acceptability in regards to the capacity of vernacular languages to express the perceived complexities of the Latin Bible and the people’s suitability of reading the Bible for themselves shifted etc, the idea of translating the whole Bible moved one window towards admissibility. The Reformation compelled the idea to switch windows pretty quickly. Today, the spectrum has moved so much that reading the Bible in Latin, unless you’re a scholar, is close to unthinkable.
If we look at even earlier periods, we find the same shifting of windows. Magna Carta was a success because the right Overton windows were suitable opened when it appeared, and it appeared because the windows were open. A historic impossibility, like having Magna Carta in the 9th century, for instance, instead of the 13th, is another way of saying that the Overton windows were closed, or the wrong ones were open.