"What is Man?" is Mark Twain's most serious philosophical and private book. He had worked on it  for a long time until he was finally satisfied with it bu still chose not to release it until after his death. Consisting primarily of a long dialogue between an old man and a young man the book argues that humans are largely mechanistic in their thoughts and actions. It is a thought-provoking work of philosophy which will make you re-evaluate this much-loved author. Anyone who thinks Twain was not a brilliant philosopher should read this book.

"The fact that man knows right from wrong proves his intellectual superiority to the other creatures; but the fact that he can do wrong proves his moral inferiority to any creature that cannot."