What a wee little part of a person’s life are his acts and his words! His real life is led in his head, and is known to none but himself. All day long, and every day, the mill of his brain is grinding, and his thoughts, (which are but the mute articulation of his feelings,) not those other things, are his history. His acts and words are merely the visible thin crust of his world, with its scattered snow summits and its vacant wastes of water—and they are so trifling a part of his bulk! a mere skin enveloping it. The mass of him is hidden—it and its volcanic fires that toss and boil, and never rest, night or day. These are his life, and they are not written, and cannot be written. Every day would make a whole book of eighty thousand words—three hundred and sixty-five books a year. Biographies are but the clothes and buttons of the man—the biography of the man himself cannot be written.
—Mark Twain, Autobiography of Mark Twain: The Mark Twain Papers, Volume 1, 220-21.
(with a nod to Maria Popova at curiositycounts)