[2] He then served as an assistant to Niels Bohr in Denmark and was later a colleague of Albert Einstein at the Institute for Advanced Study in Princeton, New Jersey.

His father, Isaiah "Jacques" Pais, was the descendant of Sephardic Jewish immigrants from Portugal to the Low Countries around the beginning of the 17th century.

[3] Pais was a bright student and a voracious reader during his early education and said he had a happy childhood and felt integrated in Dutch society.

[4] At age twelve he passed examinations to enter a Hogere Burgerschool and attended a school in Amsterdam with a five-year curriculum of basic subjects.

In the winter of 1936/1937 his career goals were defined by two guest lectures by George Uhlenbeck, professor of theoretical physics at University of Utrecht.

On February 16, 1938, Pais was awarded two Bachelor of Science degrees in physics and mathematics, with minors in chemistry and astronomy.

During the remainder of the spring term he discontinued attending classes in Amsterdam and made several trips to visit Uhlenbeck in his laboratory.

He was also influenced then by discussions with Léon Rosenfeld of the University of Liège, who was invited to Utrecht to give a colloquium in an effort to find a successor for Uhlenbeck and reported on the work he was then doing with Christian Møller on the meson theory of nuclear forces.

In November 1940 the German authorities issued a decree banning Jews from all civil service positions, including academic posts.

During his student years Pais had been involved in the Zionist movement, through which he became acquainted with Trusha (Tirtsah) van Amerongen and Tina (Tineke) Strobos, and developed a close friendship with these two women and their families.

The Germans began to gradually restrict the activities of the Dutch Jews and in early 1942 required them to wear yellow stars.

In early 1943, however, the Dutch secretary general of internal affairs, Frederiks, made arrangements for the university Jews to report to Barneveld for their own safety, where they would be housed in a chateau.

When the Germans began forcing the Dutch Jews into a ghetto in the old Jewish quarter of Amsterdam, his sister Annie and her husband Hermann complied.

His last hiding place was in an apartment with his university friend Lion Nordheim, his wife Jeanne, and her sister Trusha van Amerongen.

Jeanne and Trusha had blond hair and blue eyes and ventured out in public as non-Jews, while Lion and Pais hid in the apartment.

The same week the Americans had crossed the Rhine and cut the rail lines, making impossible their transfer to a concentration camp.

During World War II, Pais's doctoral dissertation had attracted the attention of Niels Bohr, who invited him to come to Denmark as his assistant.

In 1946, following the war, Pais was able to accept that invitation and served as a personal assistant to Bohr at his country home in Tisvilde for a year.

The technical contributions for which he is recognized include a precise definition of G-parity with Res Jost, and his treatment of SU(6) symmetry breaking.

The second concept was Pais's and Gell-Mann's theory regarding the composition of the neutral kaons, proposing that the observed states were admixtures of particles and antiparticles, having different lifetimes; this was experimentally confirmed in the following year by Lederman and collaborators.

His book The Genius of Science: A portrait gallery (Oxford University Press, 2000) contains biographies of seventeen distinguished physicists he had known personally: Niels Bohr, Max Born, Paul Dirac, Albert Einstein, Mitchell Feigenbaum, Res Jost, Oskar Klein, Hans Kramers, Tsung-Dao Lee and Chen Ning Yang, John von Neumann, Wolfgang Pauli, Isidor Isaac Rabi, Robert Serber, George Uhlenbeck, Victor Frederick Weisskopf, and Eugene Wigner.

After his retirement Pais and his third wife Ida Nicolaisen spent half their time in Denmark where he worked at the Niels Bohr Institute.