Just as Josiah Willard Gibbs was assuredly the greatest American physicist of the nineteenth century, Richard Phillips Feynman was arguably the greatest American-born theoretical physicist of the twentieth century. Feynman was among the truly great physicists of the world. Mark Kac, the eminent Polish-American mathematician, wrote (Kac 1985, p. xxv):
In science, as well as in other fields of human endeavor, there are two kinds of geniuses: the ‘ordinary’ and the ‘magicians’. An ordinary genius is a fellow that you and I would be just as good as, if we were only many times better. There is no mystery as to how his mind works. Once we understand what he has done, we feel certain that we, too, could have done it. It is different with magicians. They are, to use a mathematical jargon, in the orthogonal complement of where we are and the working of their minds is for all intents and purposes incomprehensible. They seldom, if ever, have students because they cannot be emulated and it must be terribly frustrating for a brilliant young mind to cope with the mysterious ways in which the magician's mind works. Richard Feynman [was] a magician of the highest caliber.