(1) Thomas Jefferson
The scouting report on our third president is awe-inspiring. Declarer of Independence. Purchaser of Louisiana. Designer of buildings. Founder of universities. Rewriter of the Bible. Lover of wine. Conceiver of illegitimate children. A true Renaissance man, Jefferson had all of his bases covered. He's got to be a favorite to make it to the finals.
(8) John Stuart Mill
Although he sometimes gets lost in the shuffle in discussions of history's great thinkers, Mill's genius bona fides are hard to question. He started early; at the age of three he was already speaking Greek. As a philosopher, he influentially espoused Utilitarianism, the notion that an action should be judged by how it affects the overall contribution it makes to society's happiness. As an economist, his book Principles of Political Economy was the key text in the discipline for almost 70 years. Furthermore, he was an early advocate of women's rights.
Both Jefferson and Mill had very specific ideas about how a society should function, but only Jefferson got the opportunity to lead a country. Mill had a knack for languages, but Jefferson supposedly learned Gaelic solely so he could translate Ossian's poems. Mill's economic writings laid the foundation for generations of textbooks, while Jefferson's personal library was so expansive that when Washington burned in 1814, Congress could kick start the rebuilding of the Library of Congress by buying 6,487 volumes from the former president. Both of these guys were seriously smart and cultured, but who's the bigger genius?
[See the whole bracket here.]