Karl Popper is the greatest philosopher of the 20th century. No other philosopher of the period has produced a body of work that is as significant. What is best in Popper's output is contained in his first four published books. These tackle fundamental problems with ferocious, exemplary integrity, clarity, simplicity and originality. They have widespread, fruitful implications, for science, for philosophy, for the social sciences, for education, for art, for politics and political philosophy. This article provides a critical survey of (...) Popper’s work. (shrink)