Plato, in the “Timaeus,” says that when one of the wisest men of Greece, the statesman Solon, visited Egypt, he was told by an old priest that the Greeks were like mere children because they possessed no truly ancient traditions or notions “gray with time.” In Egypt, the priest continued proudly, “there is nothing great or beautiful or remarkable that is done here, or in your country, or in any other land that has not been long since put into writing and preserved in our temples.”

Such colossal ambition coalesced under the Ptolemaic dynasty. In the third century B.C., more than half a century after Plato wrote his dialogues, the kings ordered that every book in the known world be collected and placed in the great library they had founded in Alexandria. Hardly anything is known of it except its fame: neither its site (it was perhaps a section of the House of the Muses) nor how it was used, nor even how it came to its end. Yet, as one of history’s most distinguished ghosts, the Library of Alexandria became the archetype of all libraries.

Libraries come in countless shapes and sizes. They can be like the Library of Congress or as modest as that of the children’s concentration camp in Auschwitz-Birkenau, where the older girls were in charge of eight volumes that had to be hidden every night so that the Nazi guards wouldn’t confiscate them. They can be built from books found in the garbage, like the Yiddish Book Center in Amherst, Mass., set up in 1980 by the 24-year-old Aaron Lansky from volumes discarded by the younger generations who no longer spoke the tongue of their elders, or they can be catalogued in the mind of their exiled readers, in the hope of resurrection, like the libraries plundered by the Israeli soldiers in the occupied territories of Palestine. It is in the nature of libraries to adapt to changing circumstances and threats, and all libraries exist in constant danger of being destroyed by war, vermin, fire, water or the idiocies of bureaucracy.

But today, the principal danger facing libraries comes not from threats like these but from ill-considered changes that may cause libraries to lose their defining triple role: as preservers of the memory of our society, as providers of the accounts of our experience and the tools to navigate them — and as symbols of our identity.