Karl Marx
His most important work is Das Kapital, or The Capital. It is commonly known in English as simply 'Capital.' He spent many years working on the three parts of the book.  Das Kapital describes how "capitalism" works and the problems it creates, such as division of labour, alienation and exploitation. The book has led to many arguments between those who agree with the book and those who do not. Marx's ideas have been thought of as responsible for socialist revolutions (like the Russian Revolution).