Image caption Books, not the internet, were a key source for Harry and co

In a world that is overwhelmed with ways of accessing information, we must decide what to remember and what to forget, says historian Lisa Jardine.

In a recent article about the impact of the internet, New Yorker columnist Adam Gopnik gives a particularly engaging example of the hurtling pace at which the speed of access to information is accelerating.

The first Harry Potter book - Harry Potter and the Philosopher's Stone - was, he observes, published in June 1997. At a crucial point in the plot, Harry, wrapped in his invisibility cloak, manages to get into the restricted section of the Hogwarts school library in order to scour the books it contains for vital information on the alchemical origins of the philosopher's stone.

Just one year later, in 1998, the founders of Google registered their internet and software company, and the global online search engine was born.

Today, any young reader of JK Rowling's The Philosopher's Stone would be bound to ask, turning the pages with bated-breath expectation as Harry Potter comes close to being discovered in the out-of-bounds section of the library: "Why didn't he just Google it?"

As so often, however, our sense of living in an age which is particularly vulnerable to being overwhelmed by too much information turns out to be misplaced.

Even before the invention of the printing press - when the distribution of information depended upon teams of scribes working with pen and ink in monastery libraries - the fear of too much to know, too much material too widely and swiftly disseminated, was already threatening to overwhelm our orderly sense of understanding.

Once books proliferated in printed form from the 16th Century onwards - "too many books, too little time" was the complaint of scholars like Erasmus and Descartes. Knowledge-gatherers scrambled to develop ever more-complicated ways of assembling, organising and distributing knowledge drawn from as wide as possible a range of erudite and unfamiliar sources for easy retrieval.

In 1689 a professor at the University of Hamburg with a passion for new technologies, unveiled a device for managing information overload - a purpose-built mahogany cabinet designed to hold and organise several thousand hand-written notes taken by an individual reader from the books they were reading.

Image caption A diagram of the cabinet favoured by the Hamburg professor

Along the back of the cabinet were narrow vertical posts, each headed by a letter of the alphabet. Running the length of each post was a sequence of brass plates engraved with alphabetised headings designed to capture topics of particular interest to the reader, each heading furnished with a metal hook, to which slips of paper containing information extracted from the owner's reading were to be attached, ready to be retrieved for re-use at a moment's notice.

It is not clear whether this rather cumbersome piece of equipment caught on (though apparently the philosopher Leibniz owned one) but the impetus behind it is obvious. Already by the 17th Century there was widespread anxiety that the sheer volume of available knowledge was getting out of hand - that an information avalanche was being compiled and disseminated in manuscript and print, which would overwhelm the human capacity to save and stockpile intellectual material.

The result was an explosion of instructions for note-taking, excerpting, indexing and classifying material derived from printed sources. Most of the bundles of notes, on fragments of paper and scraps of parchment, have crumbled into dust, but the printed reference volumes derived from them - running to hundreds of thousands of entries, and large, folio multi-volume editions - survive. Many of them, in their turn, carry the feverish hand-written marginal annotations made by anonymous 17th Century readers, marking items they especially valued and wanted to have permanently at their fingertips.

Under pressure to find shortcuts, it was not considered cheating to resort to compendia of knowledge in place of the sources themselves. Gabriel Naude, French intellectual and librarian, recommended frequent use of reference books - dictionaries, collections of proverbs, quotations from the classics, encyclopedias - as essential if anyone was to control the flood of information:

Image caption Internet use was in its infancy when Harry Potter first hit the shelves

"I esteem these collections extremely profitable and necessary," he wrote, "considering the brevity of our life and the multitude of things, which we are now obliged to know, before one can be reckoned among the number of learned men."

Of course I am labouring the point here to remind us that there has never been a time when mastering the sum of human knowledge has not been felt to be an impossible task. And historically there was the additional fear that the precious store of knowledge accumulating as the world grew in wisdom might be lost by natural or man-made disaster. Early modern compilers of information feared that without care for its safekeeping, information might run through their fingers like sand, lost forever.

A strong theme in the surprisingly large early modern literature bewailing the effect of too many books is not just worry at not being able to keep hold of everything a person is required to know, but this fear of loss. To 15th and 16th Century scholars, the period following antiquity - the so-called "dark ages"- had almost succeeded in obliterating classical learning forever. In the 17th Century, Europe-wide wars, civil wars and unrest had resulted in the destruction of entire archives of precious administrative documents.

Hence the potent theme of knowledge rescued from near-oblivion, which runs through early modern discussions of how to store and retrieve information reliably.

At the beginning of the 17th Century, the Palatinate Library in Heidelberg became the largest and most important collection of books and manuscripts north of the Alps. When the Catholic Habsburgs defeated the Protestant Palatine Elector Frederick V in 1620, they reduced Heidelberg Castle to ruins and carried off the contents of the library as spoils of war - by this time totalling over 3,500 manuscripts and 13,000 printed works - to the Vatican Library in Rome where they remain to this day.

Undeterred, at the Peace of Westphalia in 1648, when the Palatine territories were partly restored to the heir, Charles Louis, the new Elector Palatine set about rebuilding the great library, which once again gained an international reputation for its depth and breadth. In 1693, however, the French king Louis XIV again sacked Heidelberg, blowing up its key buildings and burning the refurbished library and its contents to the ground.

Given the vulnerability of paper and print before the arrival of the internet, it is not surprising that those who valued the transmission of information over time included oral or experiential transmission of skills in all kinds of hands-on, practical settings among the sources of lasting knowledge to be treasured.

Image caption Online knitting communities keep the craft alive

I came across one of these myself last autumn. My sister, who was visiting, remarked that she and I both had an idiosyncratic way of casting on stitches to begin a piece of knitting. It is not a technique that figures in any of the many online knitting sites - modern repositories of practical knowledge preserved and transmitted via the internet. Of course we learned our way of casting on directly from our mother - I can still recall her hands on mine as a child, directing the needles and guiding the yarn. Here is a piece of data inadvertently preserved by our shared practice, to be continued, I feel sure, into the next generation.

Knowledge is actually hard to lose nowadays. In one of my early Point of View pieces in 2007, I reflected on the fact that my twenty-something son had asked me what "darning a sock" meant. He really didn't know. Nor, when I explained, could he quite see the point. Socks were on sale in packs of three or more pairs in every supermarket. Why would you bother to mend a sock when you could simply buy a new pair?

The genuine satisfaction of having been frugal - having made the sock 'as good as new' for another season's wear Lisa Jardine in 2007 Read Lisa's PoV on darning

But of course, in spite of my son's generation's incomprehension, the meaning of darning has not been lost. Googling this week, I find there is a comprehensive Wikipedia entry recording every aspect of this traditional repair method (the first version, as it happens, was posted online in 2007) with diagrams and tips for successful execution. And for the novice, there are several YouTube videos carefully explaining how to achieve the ideal darn.

The danger today is rather that we are reluctant to let go of any information garnered from however recondite a source. Every historian knows that no narrative will be intelligible to a reader if it includes all the detail the author amassed in the course of their research. A clear thread has to be teased from the mass of available evidence, to focus, direct and ultimately give meaning to what has been assembled for analysis. Daring to discard is as crucial as safe-guarding, for effective knowledge management and transmission today.

There is all too little danger of the knowledge currently accumulating in floods - multiply-owned, stored and captured - being lost. Rather, if we are going to make sense for posterity of today's information-saturated present, one of the things we will have to learn to do is decide how to prune the evidence, and ultimately, what to forget.