Lev Landau, a Nobelist and one of the fathers of the great Soviet physics system, had a logarithmic scale for ranking theorists, from 1 to 5. A physicist in the first class had ten times the impact of someone in the second class, and so on. He modestly ranked himself as 2.5 until late in life, when he became a 2. In the first class were Heisenberg, Bohr, Dirac and some others; Einstein was a 0.5! (For reminiscences of great physicists in that generation, see From a Life in Physics.)
My friends in the humanities, or other areas of science like biology, are astonished and disturbed that we physicists think in this essentially hierarchical way. Apparently, differences in ability are not manifested so clearly in those fields. Personally, I find Landau's scheme appropriate. There are many physicists whose contributions I cannot imagine having made. James Gleick, in his insightful biography of Feynman, devotes an entire chapter to these issues and the elusive subject of genius.
I once asked a famous theorist what fraction of the population was capable of doing good work in our field. He had already thought about this question (as later became clear), and immediately answered: 1 in 100,000. Whether his answer is correct is debatable, but surely the fraction is a small number. Even taking 1E-05 as the answer, there are many such people on the planet, most of them in developing countries (20,000 in China and India combined!). Do they have access to educations commensurate with their abilities? Won't the world benefit more and more from their talents as globalization continues? The US has benefited enormously from the brain drain that brings such people to our country.
[Note added (2009): In About Science, Myself and Others, Nobelist V.L. Ginzburg notes that Landau had ranked Feynman in the 1 category (so, higher than Landau himself). Ginzburg describes many similarities between Landau and Feynman, although unfortunately the two never met in person. p.281]
