The Man of Genius, referred to mankind, is an originator, an inspired or demonic man, who produces a perfect work in obedience to laws yet unexplored.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

“The child should have the advantage of ignorance as well as of knowledge, and is fortunate if he gets his share of neglect and exposure. “The laws of Nature break the rules of Art.” The Man of Genius may at the same time be, indeed is commonly, an Artist, but the two are not to be confounded. The Man of Genius, referred to mankind, is an originator, an inspired or demonic man, who produces a perfect work in obedience to laws yet unexplored. The Artist is he who detects and applies the law from observation of the works of Genius, whether of man or nature. The Artisan is he who merely applies the rules which others have detected. There has been no man of pure Genius;” source