One of the most important events in political and intellectual history was the final collapse of the Roman Empire after the fall of Constantinople in 1453. (Some readers may think of this as the fall of the "Byzantine" empire, but that was just a name later historians gave it. The Empire called itself the "Roman Empire", as did everyone else at the time.)
In the run-up to the fall of Constantinople and especially afterwards, many Roman Empire scholars fled to Western Europe, taking manuscripts with them of ancient works that had, up to then, only been known and debated in Western Europe via secondary sources (i.e. other writers writing about what was said in them). What happened in response to these documents changed everything in the intellectual realm, ultimately transforming politics, also, as we shall see.
Up to this point, the dominant intellectual method in Western Europe was to build upon the knowledge of the past. Some pre-Socratic philosopher had had some idea, Socrates had commented on that, Plato had spun Socrates' analysis, Aristotle had responded to Plato. Onasander, Galen and others had written commentaries on Plato. Ibn Rushd (Averroës) had written commentaries on Aristotle. Aquinas had commented on other folks' commentaries on Aristotle. And so on.
Education in this age consisted in the development of scholarship – to be educated was to have mastery of a vast body of things that others had said. Thinking separately from this structure was pointless and arrogant. Thousands of scholars had spent fifteen hundred years building up this body of knowledge and ideas. How could any one man hope to replicate and improve upon that in one short lifetime? The notion only needed to be stated to be seen as absurd.
When no manuscript even existed of a work, there was no other way to know of what it might have said other than to consider what those that had read it had said about it. But when new manuscripts became available, and new Roman Empire scholars that had their own long traditions building upon and interpreting ancient works turned out to have different interpretations from those in the West, the idea arose that the best way (or at least a key part of the best way) to understand what someone had said might be to read it for yourself. Thus arose the ad fontes ("to the source") doctrine of the renowned Renaissance humanist Erasmus of Rotterdam.
Ad fontes changed everything. First, in philosophy, educated men began to read Plato and Aristotle instead of relying upon the interpretations of the scholastics. Next, in theology, ad fontes became the doctrine of sola scriptura, according to which the key truths were in the Bible, instead of coming from the traditions and doctrines of the priests. Then, following that, in science, the idea arose that one could discover truths by considering the original empirical data – going right "to the source" of scientific knowledge.
Instead of education consisting in developing scholarship, education came to consist in developing a skill, a method of interpreting original sources. (Socrates would doubtless have considered this ironic, as he was keen to distinguish between the "skill" of the artisan (the carpenter, potter or poet) and the knowledge of the wise.)
But there is only value in many educated persons being skilled in interpreting original sources if more than one interpretation might potentially be right or at least of value. So instead of there being one corpus of knowledge from which only hubristic arrogance seeks to deviate, we now have many skilled interpreters with their own separate and different bodies of claimed knowledge.
There could have been a number of political responses to this. Perhaps each different country might have its own preferred interpretation of the world, and if you didn't like the dominant ideas of your own country you were free to go elsewhere. Or countries could officially approve a set of interpretations or set of interpreters and outlaw or discourage others.
In Britain, eventually – from the Elizabethan period on – religion and later politics built upon a lesson from the German church. It was decided that provided that a core set of matters were agreed upon, people were free to adopt their own interpretation for secondary matters. In the political realm this eventually came to be what we know as English (or Lockean) liberalism. There are a set of explicit laws, to which are added norms and customs. Provided that one does not disobey the central requirements set out in law, as far as the state is concerned one is free to believe and to teach others (e.g. one's children) to believe things that most people in society do not. That may not be without social (as opposed to legal) consequence if one violates norm and custom, even whilst obeying the law. But there is great scope for obeying law and norm and custom whilst still being heterodox in many other opinions and practices.
All of this can be traced back to that key intellectual revolution created by ad fontes and caused, in some sense, by the end of the Roman Empire.
Once we grasp where English liberalism comes from, we can see immediately that certain objections to it miss the point. Some say we should not tolerate children being taught that the world was created 20,000 years ago because best analysis of the geophysical data suggests that that is false. Others say we should not tolerate children being taught that some races are morally or physically inferior to others because best analysis of the biological data and moral sources suggests that that is false. Others say that parents should not be permitted to smack their children because best analysis of the data on child-rearing suggests that children do better if they are not smacked.
All these objections miss the point of liberalism. If we really thought some priestly or political or technocratic class in our society equipped to discover a unique and robust truth about the world, the case for political liberalism would be weak – if we already know the truth about all matters, why need we tolerate Error? In our sort of society we equip people to analyse things for themselves and then, as a consequence, accept that they will interpret the data they analyse in ways that are different from the mainstream view and may well be wrong. But as the overlords – whether priests or technocrats or politicians – we accept humbly that that possibility of Error applies to us as well.