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8 October 2026
Search by Meaning
How the library finds passages that say the same thing in different words, what that makes possible, and what does not work yet.
The books here are in Latin, Greek, Arabic, Hebrew, Sanskrit, Chinese, Tibetan and a dozen other languages. Two of them can describe the same thing and share no word. A search for words will find one and miss the other.
So each page also gets a second kind of index entry. A model reads the page and writes a list of 768 numbers that stands for what the page is about. Pages about the same thing get similar numbers, whatever their language. About 6.7 million pages and 150,000 pictures have one. Comparing those numbers is what we mean by searching by meaning.
Below are two things this has done with real pages, then what it does for readers today, where it falls short, and what we plan to build. The figures were measured on 7 October 2026, and each links to its test.
Example 1. A picture finds the page that explains it
Michael Maier’s Atalanta fugiens (1618) is a book of fifty alchemical emblems. We gave the model the picture alone, with no caption, and asked which page of an English manuscript translation of the book it belonged to. The manuscript has 194 pages and no pictures.
For the picture on the left, the first page it chose was the one on the right. It reads:
“If geometry is so natural and easy for children, how does it come to pass that the "squaring of a circle" was not known to Plato himself?”
Across the 44 emblems we hold a clean picture of, the first page chosen belonged to the right emblem 34 times. Picking a page at random would be right less than once. This is a test, not yet a feature of the site. The test
Example 2. One idea in four traditions
At the end of the Republic, Plato tells of a soldier who dies, sees the dead judged, and comes back to report it.
“He saw then, in this way, the souls departing through each opening of both heaven and earth after they had been judged”
We asked for the nearest page to this one in each other tradition in a test set of 304 books. These are the three it returned. None of them mentions Plato.
“the angels of mercy raise them to the fourth heaven with honor and exaltation”
“Joined with Adharma, he goes to the realm of Yama.”
“they saw a green field that smelled sweetly and was full of beautiful flowers, and it was wonderfully clear and bright.”
The quotations are from our machine translations; follow each link to see the original page beside it. This was the best result of 25 tries, not a typical one. The typical result is under what does not work yet.
What it does today
Search inside one book
The same, limited to the book you are reading.
The Librarian
An assistant that looks up passages and answers with quotations and page links.
Ask the LibrarianPictures
Describe a picture in words, or pick one and see pictures that look like it.
Open the galleryFor AI assistants
Any assistant that speaks the MCP protocol can search the library and quote pages with their links.
Developer notesWhich models do this work is listed on the models we use.
What does not work yet
Pages we have not translated are hard to find in English.
We wrote 40 questions in English whose answer is on a Latin, German, French or Chinese page with no translation. Search put the right page in its first ten results for 4 of them. The page’s numbers were sound: searched apart from the translated pages, they found 18 of 28. Translated pages crowd the others out. A separate index for untranslated pages is being built. The test
A search for an idea mostly returns one tradition.
For 25 ideas that several traditions wrote about, the first ten results held relevant passages from 1.88 traditions on average, and from one or none for 10 of the 25. That is after search began spreading its results across traditions on 7 October; before, it was 1.68 and 15. Counting every passage, relevant or not, the first ten now come from 4.16 groups of traditions. Each tradition has its own vocabulary, and the numbers follow the vocabulary. Describing each page in two plain sentences first, and searching those, raised the average to 2.8. The test · The test of the spread
One page cannot yet suggest its own parallels.
We took 25 pages and asked for the nearest page in each other tradition. A reader who did not know how the pages were chosen judged 13 of 75 suggestions to be about the same idea. That is too few to show on a page, so we do not. The test
Some stored numbers are out of date.
In a sample of 10,888 pages, 16 in 100 had numbers that no longer matched the page’s current text. They are being found and refreshed. The test
What we plan to build
In the order we expect to build them. “Tested” means a small trial with real pages is linked on this page; “planned” means no trial yet.
1. A page for each idea
testedOne page that gathers what Greek, Hebrew, Arabic, Sanskrit, Chinese, Tibetan and Latin sources say about the same idea, each passage quoted and linked, each one checked by a reader before it appears.
2. From a picture to its text
testedEvery emblem and plate linked to the page that explains it, and every such page linked back to the picture.
3. An open test set
plannedOur questions and their checked answers, published so that anyone can test a search system on old books in several languages. The standard collections of such tests have none for historical texts.
4. Translating what readers look for
plannedCounting which untranslated books searches keep reaching, and translating those first.
5. The same passage in another book
plannedFinding where one text quotes, translates or borrows from another, across languages.
What it costs
Writing the numbers for a page costs a few thousandths of a cent, and a few hundred dollars for every page in the library. The two-sentence descriptions that help a search cross traditions would cost about $530 for every page, so we are testing them on a part of the library first.
The full list of ideas we considered, with the published work each rests on, is in our working notes. How the index is built is in the technical description, and the steps every book goes through are in how Source Library works.
How this article was made
This is an AI-assisted article. Derek Lomas set the question; AI models searched the library, read the sources and drafted the text, which was reviewed before it was published.
Every quotation is checked by machine against the scanned page it comes from and links to that page, so you can read the original yourself. The article is a living document: it is revised when a reader finds an error or the library gains a better source.
How these articles are made · Last revised 18 October 2026 · Revision history · Spot an error? Suggest an edit
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