Editor's Note: There is probably no modern scientist as famous as Albert Einstein. Born in Germany in 1879 and educated in physics and mathematics at the Swiss Federal Polytechnic School in Zurich, he was at first unable to find a teaching post, working instead as a technical assistant in the Swiss Patent Office from 1901 until 1908. Early in 1905, Einstein published “A New Determination of Molecular Dimensions,” a paper that earned him a Ph.D. from the University of Zurich. More papers followed, and Einstein returned to teaching, in Zurich, in Prague, and eventually in Berlin, where an appointment in 1914 to the Prussian Academy of Sciences allowed him to concentrate on research. In November of 1919, the Royal Society of London announced that a scientific expedition had photographed a solar eclipse and completed calculations that verified the predictions that Einstein had made in a paper published three years before on the general theory of relativity. Virtually overnight, Einstein was hailed as the world's greatest genius, instantly recognizable, thanks to “his great mane of crispy, frizzled and very black hair, sprinkled with gray and rising high from a lofty brow” (as Romain Rolland described in his diary). In the essay excerpted here, and first published in 1936, Einstein demonstrates his substantial interest in philosophy as well as science. He is pragmatic, in insisting that the only test of concepts is their usefulness in describing the physical world, yet also idealistic, in aiming for the minimum number of concepts to achieve that description. In 1933, Einstein renounced his German citizenship and moved to the United States, where he lived until his death in 1955. A recipient of the Nobel Prize in physics in 1921, he was elected a member of the American Academy of Arts & Sciences in 1924.