Trial And Error.
"Anyone who has never made a mistake has never tried anything new." - Albert Einstein

The Legacy of World War II
* Mathematics at Gottingen under the Nazis - Saunders MacLane
http://www.ams.org/notices/199510/maclane.pdf
* Banach's Life in Poland
"German soldiers barricaded two ends of a street and shot everybody between. There was, however, a possibility of escape from this recurrent problem. The Germans needed a great quantity of lice in order to produce a vaccine against typhoid and other diseases. Poles were employed to wear a box of lice on the wrist, fed by the human host. The wrist itched frightfully. The pay was that a Pole with such a box might not be shot as Saks was. Banach wore a box. More than half the Polish mathematical profession perished during the war."
* As Hilbert Saw It
"When Hilbert reached the age of 68 in 1930, he was forced to retire from teaching. In 1932, Adolf Hitler became the chancellor of Germany and a law was passed forbidding full-blooded Jews from teaching positions. This ban applied to Courant, Noether, Landau, Bernays, Born, and Franck. At a banquet, the minister of education asked Hilbert, 'And how is mathematics in Gottingen now that it has been freed of the Jewish influence?' Hilbert replied, 'Mathematics in Gottingen? There is really none any more.' The Nazi regime ended Gottingen's position as the center of the mathematical world."