Physicist Albert Einstein's letters, including one in German about his theory of relativity, are up for auction. Associated Press

When he wasn't busy scribbling out the theory of relativity, Albert Einstein seems to have spent a fair amount of time writing letters involving topics such as God, his son's geometry studies, even a little toy steam engine an uncle gave him when he was a boy.

The Einstein Letters, which include more than two dozen missives, will go up for sale on Thursday at the California-based auction house Profiles in History. Some were in English and others in German. Some were done in longhand, others on typewriters.

Albert Einstein

Amassed over decades by a private collector, the letters represent one of the largest caches of Einstein's personal writings ever offered for sale.

But more than that, they give a rare look into Einstein's thoughts when he wasn't discussing complicated scientific theories with his peers, said Joseph Maddalena, founder of Profiles in History.

"We all know about what he accomplished, how he changed the world with the theory of relativity," Maddalena said. "But these letters show the other side of the story. How he advised his children, how he believed in God."

In one letter, Einstein urged one of his sons to get more serious about geometry. In another, he consoled a friend who recently discovered her husband's infidelity. In still another to an uncle on his 70th birthday, Einstein recalled how the toy steam engine the uncle gave him years ago had prompted a lifelong interest in science.

On the issue of God, Einstein dismissed the widely held belief that he was an atheist.

"I have repeatedly said that in my opinion the idea of a personal God is a childlike one," he wrote to a man who corresponded with him on the subject twice in the 1940s. "You may call me an agnostic, but I do not share the crusading spirit of the professional atheist. ... I prefer an attitude of humility corresponding to the weakness of our intellectual understanding of nature and of our own being."

Maddalena expects each of the 27 letters to fetch anywhere from $5,000 to as much as $40,000, for a total take ranging from $500,000 to $1 million.

They are priceless, in his opinion, when it comes to having a greater understanding of the most brilliant physicist of the 20th century.

"These are certainly among the most important things I've ever handled," Maddalena said. "This is not like a Babe Ruth autograph or a signed photo of Marilyn Monroe. These are historically significant."