i think that the best answer to your question would be an essay writ en by Albert Einstein, as part an article that he wrote on Nov/26/1938 for Collier's Magazine NY.
i found it in a book call es Ideas and Opinions by Albert EInstein, check out this link: http://www.amazon.com/gp/product/0517003937/102-6772225-4301709?v=glance&n=283155