u wot mate?

Half of the phrase intellectual history is history, so let's look at the history behind your point. Both in terms of the actual facts, and in terms of how actual historians deal with them.

Islam's golden age followed upon the loss (maybe destruction) of Persian and Central Asian literature.

Actually, one of the really interesting things about the rise of Islam and the golden age of Islam is the incredibly enduring Persian presence. Some of the most famous medieval "Arabic" literature comes from Persian authors and was often written in Persian, not in Arabic. The Muslim, Jewish, and Christian scholars working at the court in Baghdad and its peripheries, in Cordoba, Toledo and their peripheries, were absolutely not creating new things out of nothing. Much like scientists and other people today, they started with the work of earlier scholars and built on it.

After the printing press costs of writing dropped, so ideas previously transferred orally were likely to appear in text then, making the Renaissance look more innovative than it was.

Know why the printing press was able to be invented in the first place? Why all that time, effort, and money was justifiable? Because people were already reading in large enough numbers for early printers to be able to expect an audience for 500, 1000, even more copies of the same book. But this doesn't mean there were no books circulating earlier.

In terms of both the Renaissance and the Reformation, scholars have put a ton of work into tracing their origins--into picking through what WAS new and innovative, in showing how people came up with their ideas. With the Renaissance, scholars talk about medieval humanism and how it was different than Renaissance civic humanism (Southern); the "Greek visitors" with the fall of Constantinople (Angold); the Burgundian renaissance/medieval harvest occurring at the same time as what we call the Italian Renaissance (Belozerskaya); and crucially, how the "Italian Renaissance" elite were doing one thing while the outlying Italian areas were still basically in 1200 (Herlihy, Klapisch-Zuber). With the Reformation, I mean, you can no long talk about Protestantism and Luther without talking about fifteenth century mysticism (Oberman); pastoral care/theology (Hamm); political reform as model (Brady); Wyclif and the Lollards (literally the entire field of medieval English literature between 1990-2010); Jan Hus and the Hussites (starting with Luther himself).

Could everyone read and write? Absolutely not. Literacy rates were rock-bottom low; mass literacy is an incredibly modern development. However, scholars are keenly aware of this. The rise of social history in the mid-ish 20th century cast a lot of attention on getting to the lower layers of society, beneath the elite political narrative; the Annales school and then the rise of cultural history helped focus attention not just on the structures of society but on how and what people thought. So scholars have made and continue to make Olympian efforts to scrape through sources to uncover--indeed--what people thought. They've used sermons and penitentials to track what concerns priests were learning to respond to (Bast, Hamm); combed inquisition and trial records to pick up patterns in what people thought about magic and "folklore" topics (Wilby, Bever); immersed themselves in the material culture of everyday life (Bynum and everyone who wants to be her, because we all want to be Bynum); and heavily, heavily debated the extent to which peasants involved in political protests or religious movements fully understood that Big Issues involved (Cohn, Blickle, Stayer, Justice, Stock...).

In other words: historians understand that no one comes up with ideas out of nowhere. That is literally the entire field of intellectual history: where did these ideas come from and what were their effects. This doesn't discredit the innovators; it makes them more important--an idea so wildly out of sync with its own era would have absolutely no traction. Two people invented calculus at the same time; liquid-fuel rocketry, three people. The ideas were in the zeitgeist.

Yet another example is academia. Academics are rewarded for writing. Nuns, bakers, judges were not. Much of what we attribute to academics might have been public knowledge, or the invention of some nun.

Excuse me, what do you mean nuns were not rewarded for writing? Perhaps you did not know that nuns basically invented education for girls in the western world. Some of the most famous women of the Middle Ages were either nuns or related religious women (beguines and tertiaries), and guess what--most of them were authors. Hildegard of Bingen and Birgitta of Sweden aren't just famous today; they were two of the most enduringly influential women in the Middle Ages. Guess what--nuns and writers. But we don't have to look at just the standouts. Not all, but many monasteries cultivated rich intellectual lives through reading and writing. Cynthia Cyrus and Alison Beach on the incredible extent of women's scribal activity; Sara Poor, Anne Winston-Allen, Anneke Mulder-Bakker thinking through how scribes aren't just "copyists," but influential authors and shapers of the literary world in their own right; John Van Engen, John Coakley showing how it wasn't just the richest or most educated who were both consumers and developers of medieval religious/intellectual life.

In fifteenth century German, the cultural phenomenon of Meistersang was predicated on the idea that its practitioners were average artisans. Far from being "not rewarded" for their writing as bakers and clerks, they don't even sign their songs as Meistersinger--they do it as "kuchner", as "barwierer." Although eventually much of the focus came to rest on the musical end of things, the words of their poetry also show keen engagement with intellectual currents of the day.

Books by authors though of as imprudent systematically disappear.

There is absolutely no question that we have lost tons and tons of books. Some of these were indeed consciously destroyed. We know about Celsus through his opponent Origen. We don't give Origen credit for the ideas of Celsus. In fact, one of the key characteristics of intellectual development and writing for millennia was the acknowledgement of earlier authority. Much like scholars today have footnotes or I'm parenthetically citing ONE EXAMPLE OF the scholars responsible for the research points above, antique, medieval, early modern writers built their arguments on citation to earlier authority. Your argument is self-defeating: we know about Epicurus; we know a damn good bit about his ideas; we named a philosophical "movement" after him.

And despite this "prudishness", Catullus' Carmen 16 somehow made it down through the centuries, which probably tells you all you need to know about that.

There are many good and constructive ways to critique intellectual history. Sometimes I think the field strays too close to being history of philosophy, coming unmoored from its necessary roots in social context. Modern scholars, working in a non-theist context regardless of personal religious views, often tend to strip out the religious context of earlier eras (Christian or Islamic; Jewish writers and texts are often studied within a Jewish faith/intellectual perspective because medieval Hebrew is really, really hard to learn).

But your criticisms are unfounded and uneducated because they are unresearched. The people you accuse of perverting intellectual endeavour aren't the ones doing so--you are.

In addition to what /u/sunagainstgold says, I should remark that you have a very erroneous impression of how texts get transmitted. This is not uncommon among polemicists who don't really have training in philology or textual criticism, but as with them the crux of your statement rests on the textual tradition, and a twisted understanding of the textual tradition will do you no favors.

You seem to be under the impression that texts are lost in cataclysms or through the intentional suppression of authorities, a sort of censorship largely unknown in the ancient world. This is fallacious. If the Library of Congress went up in flames right this moment, apart from a relatively few internal cataloging texts and a very few rare manuscripts, practically nothing of value would be lost. The contents of the library could be easily recovered, as the library at Alexandria's contents were, from other collections. Nearly everything in the Library of Congress can be found elsewhere, and the same has been true generally throughout antiquity and the Middle Ages. Likewise, the suppression of texts by authorities is well-known to be ineffective, except in providing an obstacle--never insurmountable--to a relatively small group's ability to access such texts. Consider that literally thousands of books are currently banned, have been banned, or were and are placed under restricted access, in the People's Republic of China at the present moment. Currently, that list includes books like Green Eggs and Ham (banned 1965, lifted 1991). Are we to suppose that all copies of Green Eggs and Ham, even within China, suddenly disappeared? That printing of Green Eggs and Ham suddenly ceased? That interest in Green Eggs and Ham dropped off so suddenly that the text has been effectively forgotten? None of these is true, and all of them are laughably absurd.

Texts are lost because they stop being copied. Period. They may stop being copied for a number of reasons, but usually it's the result of a decline in the ability to produce texts or distribute them, or otherwise a loss in popularity of that particular work. Paper was expensive, and prior to the printing press books were labor intensive and difficult to produce--that in antiquity the cost of books appears to have been relatively low is a testament to the ability of the Roman economy in particular to mass-produce even luxury goods at a scale totally unimaginable before. In an economy like the Roman one at the height of its prosperity texts could be produced widely and distributed all over. Not so as economic conditions began to decline. The vast majority of ancient texts that have been lost were already lost at the end of antiquity, as the conditions to produce books and the demand for them rapidly declined. Further texts were lost in the Middle Ages, despite the near-fanatical efforts by most abbots to preserve any and every scrap of writing in monastic libraries, as resources became even scarcer and these texts became consigned to preservation collections rather than being actively distributed. Consider, after all, your own example: Plato. You seem to be under the impression that Plato was readily preserved throughout antiquity. In the Byzantine east Plato never lost his relevance, but Plato was completely lost in the west. Knowledge of Greek disappeared in the west as Greek lost its place as a literary language among the educated, and Greek texts stopped being copied (except in Ireland) in favor of Latin texts that could actually be read and which copyists were more justified in spending precious time and paper copying--a perfect example of the loss of textual material for practical reasons. Aristotle was preserved through Latin translations of some of his writings, but Plato, despite his later significance in the Renaissance, was lost. If Plato was a representative of some sort of philosophical orthodoxy supplanting a suppressed philosophical tradition, then we should expect the opposite to be true, that knowledge of Plato and even Greek would continue in the west despite all the hardships, as Plato was being prioritized over translations of Aristotle and educational texts like Caesar or Virgil. This is manifestly not the case.

Moreover, when texts stopped being read there was no longer any reason to copy them, as demand dropped. This is precisely what happened to Greek in the west, and we see it even in antiquity. Caesar, for example, never formally published his poetry, and only distributed it privately to his friends and the educated. As a result already by Tacitus' time it was probably scarce, and had disappeared entirely by late antiquity. By your logic one would expect that political interests would have naturally promoted Caesar's writings over more supposedly "subversive" poets like Ovid, yet Ovid was preserved and hailed throughout antiquity and the Middle Ages as a master of elegy and the Hellenistic epic. Even a brief period of unpopularity could endanger even the most acclaimed texts. /u/sunagainstgold mentions Catullus, whose popularity declined briefly under the Antonines despite being hailed by all educated writers as a great poet. By late antiquity those few decades had nearly wiped out Catullus' poems, and only a single manuscript survived into the Middle Ages, despite the universal lament of commentators. By contrast, Martial, the content of whose poems was every bit as bawdy and subversive as Catullus', was widely copied and preserved continuously, having never experienced such a disastrous decline in popularity. Literary taste changes over time, and when materials are costly and difficult to produce priorities must be assessed.

As a sort of postscript, describing Epicurus as a representative of some sort of "counter-culture" is utterly ludicrous. Epicureanism was literally, without question the single most popular philosophical school in the Hellenistic and Roman world, and it was not until relatively late in the Principate that Stoicism and eventually Neoplatonism (which has little to do with Plato's own Academic school) gained enough of a foothold to oust Epicurus' dominance. Epicurus was literally the first, and for a long time the only, philosopher whose writings were introduced into Roman society, and even after the introduction of new philosophical schools Epicureanism still remained dominant. Moreover to call him a "counter cultural atheist" is misunderstanding, or even misrepresenting, the way ancient religion worked. Epicurean authors like Virgil or Horace wrote about the gods and addressed them. Does Cicero's Academicism somehow automatically make him more ritually pleasing, in a society in which public ritual coexisted with mystery cults, magic, and other private expressions of ritual activity? Describing Epicurus thus is a little like saying the pope is countercultural because he maintains the doctrine of transubstantiation when nearly every other Christian denomination in the world has rejected it