When Arthur Dent asked Prak the Truthful what the Ultimate Question of Life, the Universe and Everything, to which the Answer is forty two, actually was, Prak replied, "The Question and the Answer are mutually exclusive. Knowledge of one logically precludes knowledge of the other. It is impossibe that both can be known in the same universe. If it happened it seems that the question and the answer would just cancel each other out, and take the universe with them, which would then be replaced with something even more bizarrely inexplicable. It is possible, however, that this has already happened, though there is some some uncertainty about it..."

There is a theory that the last possibility was cooked up by a wily editor of the Hitch Hikers Guide to the Galaxy to sow the seeds of uncertainty and therefore increase sales of the Guide.

There have been many theories from fans in an attempt to explain why the number 42 was chosen, and what secret meaning it may hold.

Some propose that it was chosen because 42 is 101010 in binary code, others have pointed out that light refracts through a water surface by 42 degrees to create a rainbow, and others have commented that light requires 10^-42 seconds to cross the diameter of a proton.

Other fans say that 42 is Adams' tribute to the indefatigable paperback book, and is the average number of lines on an average page of an average paperback. Another common theory is that 42 refers to the number of laws in cricket, a recurring theme of the books.

Some fans have thought that "what do you get when you multiply six by nine?" was the Ultimate Question. After all, it was Arthur who has been led to reveal this answer; Arthur was indeed from Earth; the Earth was the giant computer built to work out exactly what the Ultimate Question was; and it is a bona fide question*. Six times nine does indeed equal 42 when one calculates in base 13. Adams responded thus: "I may be a sorry case, but I don't write jokes in base 13."
* However, Arthur is a descendant of the Golgafrinchans, not of the australopithecenes inhabiting ancient Earth. They died out after the Golgafrinchan B Ark crashed in ancient Britain, two million years ago.

Douglas Adams revealed the reason why he chose forty-two in this message.

"It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk, stared into the garden and thought '42 will do'".

He further explained in January 2000, in response to a panelist's "Where does the number 42 come from?" on the radio show "Book Club", that he was "on his way to work one morning, whilst still writing the scene, and was thinking about what the actual answer should be". He eventually decided that it "should be something that made no sense whatsoever – a number, and a mundane one at that". He arrived at the number 42, completely at random.

Despite Adams stating that he chose the number 42 at random, and that it has no hidden meaning, and his collaborator on 'The Meaning of Liff' and two Hitchhiker's fits, John Lloyd, saying that Adams has called 42 "the funniest of the two-digit numbers", fans still speculate about the 'true' meaning of the number.