What is up for sale is Popper's mental map: his working library, the books and journals he brought with him to Britain when he arrived in the Forties and to which he added almost daily until his death aged 92 last September. It is the furniture of a powerful mind, formed in that Mecca of philosophers, Vienna.
"Books were a part of my life before I could read them," he once recalled. This was a man whose father, a Jewish lawyer, translated Horace in his spare time, and whose editions of Plato, Bacon, Descartes and Schopenhauer - the most lucid German philosopher, according to Sir Karl - are now for sale.
For two years in the Twenties, Popper was apprenticed to a cabinet- maker, but his French polishing "suffered from my preoccupation with epistemology". The result in 1932 was The Logic of Scientific Discovery - his rejection of Wittgenstein and the rest of the Viennese Circle with the argument that true science consists in advancing propositions that cannot ever be confirmed but which can be falsified.
After spending the war years in New Zealand, Popper came to the London School of Economics in 1946 and was knighted in 1965. Philosophy has recently been enjoying a vogue, and at his death Popper's reputation was riding high. His strong anti-Marxism made him a totemic figure for the Thatcherites.
The force and clarity of Popper's attack on "historicism" - any theory, any religion, which supposes it knows what is going to happen in future - ensured that such books as The Open Society and its Enemies remain on undergraduate reading lists in philosophy and social science.
Yet Popper's influence is questionable. His description of science is one that few practising scientists have ever recognised in their own work. His radical scepticism about the possibility of secure knowledge of anything - all we can do is make tentative propositions liable to be overturned at any moment - is an impossible recipe for the way most of us live our lives. His dislike of philosophy as wordplay never succeeded in dulling the fascination of Wittgenstein, who in a spat at King's College Cambridge once threatened Popper with a poker.
There will be a strong American and Germanic presence among the bidders. The Californians have become keepers of the history of that central European intellectual diaspora that pushed such seminal thinkers as Friedrich von Hayek to London in the late Thirties and Forties.
The Hoover Institution on the Stanford University campus in California already holds Popper's papers - he sold them in 1985 but (a stipulation the Churchills were not making when they were selling Winston's papers) he also insisted they remain available on micro-film in London: they can be consulted at the British Library of Economic and Political Science.
The sale does, however, include two letters, both in German, to Popper from Albert Einstein. One, at up to £4,000, expressed Einstein's appreciation of The Logic of Scientific Discovery. The other, on a problem in quantum mechanics,from 1935, is priced for guidance at between £8,000 and £10,000.
But it is the library, being sold as a single lot, to which connoisseurs of the life of the mind will be drawn. Here are bound volumes of Annalen Der Physik from those extraordinary years when Einstein published his first, revolutionary papers; inscribed copies of works by friends such as Ernst Gombrich the art historian, and a 1936 presentation copy of A J Ayer's Language, Truth and Logic - "To Carl Popper, who could have made it into a much better book."
Sotheby's specialist in charge of the sale, Peter Beal, says the library would be a coup for any academic institution: scholars would beat a path to its door, and at £150,000-£200,000 it is a cheap route to distinction.
That Popper did not leave his books to the LSE is "a disappointment", says its archivist, Dr Angela Raspin, diplomatically. She points to a growing interest in reconstructing the libraries of famous scholars and thinkers. "It is important to establish what someone actually used. In Popper's case it is the marginal notes, the scribbles on the text: that's how you trace how a particular idea or theory got going."
If the Popper sale goes well it could send a financial signal to other philosophers. The widow of Sir Freddie Ayer, a major figure philosophically at odds with Popper, owns an extensive collection of his books.
Academic carrion-birds have started to gather round Sir Isaiah Berlin; Michael Ignatieff is writing a biography. The word at Oxford is that at least his papers will go to All Souls.