Samuelson was likely the most influential Economist of the later 20th century. In 1996, when he was awarded the National Medal of Science, considered to be America's top science-honor, President Bill Clinton commended Samuelson for his "fundamental contributions to economic science" for over 60 years. Samuelson considered mathematics to be the "natural language" for economists and contributed significantly to the mathematical foundations of economics with his book Foundations of Economic Analysis. He was author of the best-selling economics textbook of all time: Economics: An Introductory Analysis, first published in 1948. It was the second American textbook that attempted to explain the principles of Keynesian economics, and the first one to be successful. It is now in its 19th edition, having sold nearly 4 million copies in 40 languages, including Russian, French, Greek, Slovak, Chinese, Portuguese, German, Spanish, Polish, Japanese, Czech, Vietnamese, Hungarian, Indonesian, Swedish, Croatian, Dutch, Turkish, Hebrew, Italian, and Arabic. James Poterba, former head of MIT's Department of Economics, noted that by his book, Samuelson "leaves an immense legacy, as a researcher and a Teacher, as one of the giants on whose shoulders every contemporary Economist stands".