A unique and very personal letter written by German physicist Albert Einstein to his close friend Michele Besso, discussing about various topics including God, is expected to fetch over USD 60,000 at an auction in the US. Datelined from Princeton, April 15, 1950, the letter by Einstein melds the philosophical with the scientific and gives insight into the disciplined scientific mind of the great physicist. In the letter, Einstein writes about wide-ranging content spanning from the spiritual - referencing God as "Him" - to his Unified Field Theory, with equations from the theory in his hand, that he believed neutralises his critics, according to Nate D Sanders auction house. "There is one thing that I have learned in the course of a long life: It is devilishly difficult to get closer to 'Him', if one does not want to remain on the surface," Einstein wrote. "There is no such thing as a 'particle' in the strictest meaning of the word," Einstein writes taking aim at quantum physicists. He even tackles the tricky issue of knowledge itself writing "no guarantee that it will ever be possible to know whether the theory is 'true'". The two paged letter is signed as "Affectionate greetings. Your A.E."The letter measures 8.5 x 11 inches and is accompanied by the original envelope postmarked from Princeton on 16 April 1950. The online auction bidding will end on September 28.