Walt Whitman has been claimed as the first "poet of democracy" in the United States, a title meant to reflect his ability to write in a singularly American character. An American-British friend of Walt Whitman, Mary Whitall Smith Costelloe, wrote: "You cannot really understand America without Walt Whitman, without "Leaves of Grass" ... He has expressed that civilization, 'up to date,' as he would say, and no student of the philosophy of history can do without him." Andrew Carnegie called him "the great poet of America so far". Whitman considered himself a messiah-like figure in poetry. Others agreed: one of his admirers, William Sloane Kennedy, speculated that "people will be celebrating the birth of Walt Whitman as they are now the birth of Christ".