Nearly 70 years ago, a group of American soldiers puzzled by a scientific article they read while in the Philippines during World War II sought to clear up their confusion with the help of none other than Albert Einstein.

Less than a month after Sgt. Frank K. Pfleegor dashed off a letter to the 20th century's greatest mind came a reply: A typed, one-page letter from Einstein himself. In the letter, now being offered for sale by Pfleegor's survivors for $40,000, Einstein humbly clarified his position and explained that “space should be looked at as a four-dimensional continuum.”

“Dear Sir: I see from your letter of April 17th that the attempt of my last publication was not reported in an adequate way,” Einstein wrote in the previously-unpublished letter, dated May 11, 1945. “I have not questioned there that space should be looked at as a four-dimensional continuum. The question is only whether the relevant theoretical concepts describing physical properties of this can or will be functions of four variables.”

Einstein's letter enlightened Pfleegor and his pals in the 26th Photographic Reconnaissance Squadron who had interpreted a late 1944 article in Science Digest to suggest the physicist was rethinking his 1915 theory of a four-dimensional universe to say there were as many as eight. Writing on letterhead from the Institute for Advanced Study at the School of Mathematics, where he worked after settling in New Jersey following his exodus from Europe in 1933, Einstein admitted he, too, was still puzzled by the weighty concepts of space and time.

"The question is only whether the relevant theoretical concepts describing physical properties of this can or will be functions of four variables."

- Albert Einstein, May 11, 1945

“I have investigated the possibilities of this kind in the last years but my respective results seem to me not very encouraging,” the letter continues. “For the time being I have returned to ordinary differential equations [from General Relativity] with dependent variables being simply functions of the four coordinates [space-time]. What the future has in store for us nobody can foretell. It is a question of success."

The original letter to Einstein, dated April 17, 1945, has been published and remains in the Einstein Papers at Jerusalem University. Researchers had assumed Einstein simply never replied; the letter’s existence was known only to the Pfleegor family, which has put it up for sale through the Pennsylvania-based historical document specialists The Raab Collection.

The collection’s vice president, Nathan Raab, told he expects a great deal of interest in the “important piece of scientific history” showing a personal side of the 1921 Nobel Prize winner.

“Anything of Einstein that’s scientific in nature is uncommon,” Raab said Thursday. “When you take that to include his theories of space-time, the four dimensions of space and how that wraps into his later research, you’re dealing with something that’s very, very uncommon.”

Pfleegor’s correspondence with Einstein was noted in a 1945 issue of the military’s “Stars and Stripes” publication, saying that “no less a figure that the great Albert Einstein” had replied to the soldier’s inquiry.

“In our tent we usually spend our evenings discussing various scientific topics,” Pfleegor wrote. “Tonight we attempted to tackle a problem from the Nov. 1944 issue of Science Digest, entitled ‘Einstein’s At It Again.’”

Science Digest's original article, according to Pfleegor’s letter, was “not very enlightening,” so he asked for further clarification.

“Some of us stick by the single four-dimensional space,” it read. “The rest say it is made up of two spaces of four dimensions. Why not three of four dimensions etc.? We would appreciate an answer.”

Pfleegor's heirs wish to remain anonymous, said Raab, who added that it is common for families of people who corresponded with famous historical figures to not realize that the letter is, in fact, a lost or unknown historical treasure.

“The condition is such that it’s very clear they cared for it, it was a meaningful piece for them,” Raab told “It’s a powerful piece. I expect a great deal of interest, not only from buyers but also from the scholarly community. This is a touching story, it shows a different side of Einstein.”