This is an extraordinary biography of an extraordinary man.
The author, himself a distinguished physicist, calls him ''the freest man I have ever known.'' He was a man who was both a great scientist and a great human being. Dr. Pais has dealt imaginatively with this duality by writing a two-track biography - almost two books in one - which deals simultaneously, and distinctly , with this free thinker as a scientist and as a human being.
This makes the book valuable both to the scientifically literate and to lay readers.
Einstein was the most important physicist to emerge soQsx4 his century. Pais gives an authoritative assessment of his professional achievements - leaving any definitive judgment of their value open, as is proper. The quest for a unified theory of physical nature, to which Einstein devoted his later years - a theory based firmly on causation rather than chance - has yet to be proved futile. Einstein may well have been right when he said it would take physicists of the 21st century to fully appreciate his work.
Essential as this assessment is for understanding a man for whom physics was a way of life, nonscientists may find this part of the book hard going. For them - and for scientists, too - there are other sections of narrative biography which portray Einstein the man, with a less esoteric account of his importance as a scientist. These sections can easily be followed by their italicized headings.
Einstein had a deep commitment to human freedom and dignity. He identified his own sense of well-being with that of what he called his ''Jewish brothers,'' to the point of being virtually a Zionist. At the same time, he was fanatical in protecting his own privacy. He needed peace and space for independent thought. Yet he had to live with the fact that he was a livIng leoend - a man who could pat a little girl on the head as he passed her on the street and have her remember it ever after.
Having known Einstein, Pais shows sensitively how these various elements in Einstein's thinking shaped his life and work. He writes critically but sympathetically, and with some biased. Fundamentally, Pais admires Einstein in the way people who have become outstanding in a field admire the great workers who inspired them in their youth.
It would take a historian with no such emotional attachment to produce a definitive biography of Einstein the man. But this cannot be done without a definitive assessment of Einstein the scientist. Perhaps we must await the next century's assessment to see this intellectual giant objectively.
Meanwhile, we can join Pais in appreciating the title quotation as Einstein's most suitable epitaph. Referring to his belief that the universe is mysterious not because it is tricky but because it is lofty, Einstein said, ''Subtle is the Lord, but malicious he is not.''