A rediscovered archive could shed new light on the work of renowned philosopher, Ludwig Wittgenstein.
Professor Arthur Gibson, from the University of Cambridge, has been examining books and papers which disappeared from public view in 1941.
He believes that one of these could be the 'pink' or 'yellow' book Wittgenstein's pupils thought existed.
He said there was also a handwritten Brown Book which differs from the version that was published.
Wittgenstein, who taught philosophy at Trinity College in Cambridge, was a prolific writer but published very little.
The work now being examined has been described by Prof Gibson as "entirely original philosophy whose existence, apart from the Brown Book was totally unknown to scholars".
It dates mostly from Wittgenstein's "middle period", November 1932 to July 1936 and, like most of his work, was never published or made public in his lifetime.
When his scribe, Francis Skinner, died in 1941, Wittgenstein packed the papers into two boxes and sent them to a former pupil, Reuben Goodstein.
In the 1970s Goodstein gave the archive to the Mathematical Association. He had been president of the institution during the 1950s.
It was later loaned to Trinity College and it was not until three years ago that Prof Gibson began working through the material at the request of the association.
The boxes contained handwritten exercise books, lecture notes and papers amounting to more than 150,000 words.
"This archive is at least as important as the Blue and Brown Books which were published after Wittgenstein's death and has other important unpublished manuscripts," Prof Gibson said.
Among these is one with a pink cover containing some "unknown narrative and many visual illustrations". Prof Gibson believes that this is the hoped-for 'pink book'.
Another "significant" find was a handwritten version of Wittgenstein's Brown Book with a revised opening and an extra 60 pages.
"It's a unique, new version that no-one has ever seen," Prof Gibson said.
"It appears to be ready for publishing as much of it is fair copy complete with instructions for the publisher."
Prof Gibson said Wittgenstein was frequently uncertain about whether he was writing one or a number of books.
He said that the archive also helped to clarify the closeness of the relationship between the philosopher and Francis Skinner, with whom he lived in Cambridge.
"Many people assumed that Skinner was simply one of many students working as note-takers for Wittgenstein.
"But these handwritten manuscripts make it clear that they were working side-by-side, with Skinner taking dictation and Wittgenstein making his own amendments in the margins and on facing pages," he said.
Prof Gibson hopes to publish the contents of the archive later in the year.
He said it would be a fitting tribute to Wittgenstein who died 60 years ago, on 29 April 1951.