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 his 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 nor day. These are his life, and they are not written, and cannot be written.

– Samuel Clemens, Autobiography of Mark Twain, Volume 1