Eureka! (as the ancients put it)

If you must write a book, and have no access to a computer, you really should start on a blank sheet of paper. But paper, or papyrus, was in short supply when a scribe named John Myronas, was compiling a prayer book, 778 years ago. So he took some old books that nobody seemed to need any more, scrubbed off the text, and recycled the pages.

His prayer book, or Euchologion, is moderately interesting to students of ancient manuscripts. But with all due respect to Myronas, it is nowhere near as significant as the old texts that he wiped out, whose traces are just about legible in the parchment. Those hidden works have now turned his book into one of the most valuable and extraordinary literary properties in the world.

To the astonishment of experts, the old Greek prayer book has thrown up yet another unique buried treasure.

It was already known that, hidden in its pages were treatises by Archimedes - hence its name, the Archimedes Palimpsest - and parts of a speech that an Athenian politician delivered 2,330 years ago. This time, digital technology has uncovered an essay on Aristotle written approximately 1,800 years ago - the third hidden jewel to emerge from the 174-page prayer book.

"The extraordinary thing is no matter how much you put into this book in terms of energy, knowledge and brain power, you always seem to get more out," William Noel, who has led the team that have been examining the book, said in a webcast given to celebrate the discovery.

The new find is a commentary written in Ancient Greek on Aristotle, who studied philosophy in Athens, under Plato, and tutored Alexander the Great. He wrote a seminal work called Categories, of which the new discovery is a critique. Its author is thought to be a philosopher named Alexander of Aphriodisias.

Aristotle's Categories has served as the foundation for the study of logic throughout Western history, according to Reviel Netz, the professor of ancient science at Stanford University. "The philosophical passage in the Archimedes Palimpsest is now definitely identified as a relatively early commentary to Aristotle's Categories," he said.

Dr Noel added: "There is no more important philosopher in the world than Aristotle. To have early views in the second and third century AD of Aristotle's Categories is just fantastic."

One newly translated passage speculates whether the category "footed" applies only to animals, or whether it includes objects, such as a bed. The writer argues: "For as 'foot' is ambiguous when applied to an animal and to a bed, so are 'with feet' and 'without feet'. So by 'in species' here [Aristotle] is saying 'in formula'. For if it ever happens that the same name indicates the differentiate of genera that are different and not subordinate one to the other, they are at any rate not the same in formula."

We now know that the scribe Myronas vandalised at least five very old books in putting together his prayer book. They included the only surviving written work by the Athenian leader Hyperides, and a collection of seven essays by Archimedes. He also pirated a liturgical book and two other books that have not been identified.

But if John Myronas seems like one of the great literary vandals of all time, there were extenuating circumstances. He lived through a very difficult period in a city that had been gratuitously wrecked. This was the fault of the misnamed Pope Innocent III, who launched the Fourth Crusade in 1204. The Crusaders assembled in Venice, but could not raise the necessary cash to sail to their planned destination, Egypt. Instead, egged on by the greedy burghers of Venice, they headed for Constantinople, the richest city in Europe, and plundered it. One witness lamented: "How shall I begin to tell of the deeds wrought by these nefarious men ... these wrathful barbarians, vomiting forth bile at every unpleasing word."

The smouldering wreckage of the city they left behind lost its position as the world centre of the Greek Orthodox religion, and there was no one to take proper care of the treasures that had been stored for centuries in its ancient libraries. Living amid the wreckage, Myronas can perhaps be forgiven for thinking that nobody needed old works by Greeks who had been dead for years.

The prayer book he created in 1229 belonged for many years to the monks of St Sabas monastery, on a hilltop a few miles east of Bethlehem. No one knows how it got there, but there was a record that the monks had it in their possession in the 16th century.

At some point, it was taken back to Constantinople, to the Metochion, or daughter house of the Holy Sepulchre, which housed hundreds of manuscripts belonging to the Greek Patriarch. In 1840, a visiting biblical scholar, Constantine Tischendorf, spotted that there was hidden writing in the parchment, without identifying the author. He was interested enough to remove at least one leaf as a keepsake. His estate sold it to Cambridge University Library in 1879.

In 1899, a Greek scholar named Papadopoulus Kerrameus undertook the job of compiling a catalogue of all the manuscripts in the Patriarch's collection. He, too, noticed the older writing under the text of the prayer book, but did not recognise its author. He did, however, transcribe a few lines. These few lines were enough to alert a Danish scholar named John Ludwig Heiberg, who was one of the world's foremost authorities on Archimedes, who hurried to Constantinople in 1906 to examine item No 355 in the Patriarch's collection. He hit gold dust. He found all the seven treatises by Archimedes that were concealed in the parchment, three of which had been lost partially or completely lost for centuries. Heiberg was already working on a book on Archimedes. The finished version included photographs of the Constantinople treasure, and a list of his latest finds.

Archimedes is of course known as the Sicilian who leapt out of his bath and ran naked down the street shouting "Eureka". Well he might, because if the old tale is accurate, he had hit upon one of the basic principles of physics.

King Hiero of Syracuse had ordered a new solid gold crown, but when it arrived, he suspected that the goldsmith had used silver with a gold coating. He asked Archimedes to find what the crown was made of, without damaging it. Archimedes could weigh the crown, but could not measure its volume, which he needed to do to test its purity - until he noticed how the bath overflowed as he lowered himself into the water, and realised that if he immersed the crown in water, he could measure its volume by the amount of water it displaced.

The other famous story about Archimedes, who was born around 287BC, is that King Hiero was stuck for an effective method of removing rainwater from the hulls of his ships, so his ingenious courtier designed a machine consisting of a hollow tube containing a spiral that could be turned by a handle at one end. When the lower end of the tube was placed into the hull and the handle turned, water was carried up the tube and out of the boat.

Scholars knew that Archimedes was a man of great wisdom and curiosity, but they did not know he had written a treatise called The Method of Mechanical Theorems until Heiberg photographed and translated it. The palimpsest is also the only sources for passages in another treatise, called Stomachion, and the only source for the original Greek text of Archimedes's essay "On Floating Bodies".

That discovery was enough to make the manuscript an almost priceless asset, and may have tempted somebody to steal it. For almost a century, it vanished, until a French family put it up for auction at Christie's in New York in October 1998. The family claimed to have owned it since the 1920s. If so, they took very little care of it. Three pages of Archimedes's manuscript that had been photographed and transcribed by Heiberg were missing, and have not been found. The book has also been severely attacked by mould, which had destroyed whole areas of text. Also, bizarrely, some time after 1929, someone had made four paintings of the Evangelists over the top of the prayer book text, and therefore over the older texts.

The Greek government claimed that it was stolen property and took out an injunction to prevent its sale, but a court permitted the auction to go ahead. The purchaser was an anonymous American who deposited it at The Walters Art Museum, Baltimore, for conservation and study. Dr Noel, the curator of manuscripts there, said that when it first came into his care he feared it was a "write-off".

But in 2002, using modern imaging technology, the palimpsest yielded its second great secret, the only known manuscript of a speech by Hyperides, a contemporary of Aristotle who was executed for leading a rebellion against the Macedonians a year after the death of Alexander the Great. The pages are damaged and difficult to read, but it appears to have been a political speech.

There could be other fragments of his speeches in the parchment.

Alexander of Aphrodisias is a more recent character who is regarded as one of the best of the ancient commentators who kept Aristotle's philosophy alive. The dedication to two Roman emperors on the frontispiece of one of his books suggests he was writing between AD198 and AD209.

Roger Easton, the professor of imaging science at Rochester Institute of Technology, said yesterday that the new discovery involved digital imaging techniques which pushed "the limits of available technology". As Professor Easton was deploying the method hoping to uncover more about the texts they already knew they were there, he realised he had stumbled something new. "Even though I couldn't read Ancient Greek, just the fact that I could see the words gave me the shivers," he said.

Dr Noel told the BBC: "I am at a loss for words at what this book has turned out to be. To make these discoveries in the 21st century is frankly nutty. It is just so exciting. We have one book that contains three texts from the ancient world that are absolutely central to our understanding of mathematics, politics, and now philosophy."