This study offers an accessible introduction to the life and work of this extraordinary thinker, including his often-neglected Postscript on scientific method published in three volumes in the 1980s. It charts the development of Popper's philosophy and shows his unfailing political commitment to humanism and enlightenment. At the centre of Popper's thought stands rationality and a strong belief in the power of the human mind to change things for the better. Rationality thus serves as a guide both in his philosophical considerations and for his political views.