This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. The book shows that the most important part of Popper's philosophy is his advance towards biology and evolution. It concerns philosophy, only with biology in so far as it sheds light on philosophy, it is immaterial for the present argument whether one takes one's stand with the 'New Synthesis' or in what sense one considers oneself to be a neo-Darwinian. Positivism claimed to contain two certainties. First, the certainty that all philosophical problems were scientific ones; and second, the certainty that the first certainty could be known with certainty. The chapter shows that Darwin's argument was only one of many arguments against religion and Darwin has taken his place among many secularisers of human thought and knowledge. Equally telling and equally signal was the failure of Positivism to account for the growth of knowledge such as it come to be experienced during the twentieth century.