Sir Karl Popper was a major thinker of the twentieth century, one who – as Anthony O'Hear writes in his new Foreword – 'has had a beneficent influence on those who have come under the spell of his thought and of the inimitable prose in which he articulates it'. It is now twenty-five years since Popper died, and thus seems – after a quarter of a century – an apposite moment to revaluate his impact, significance, and influence. The several chapters in this classic volume focus on many key elements of Popper's thought and philosophy. They are by no means uncritical, but afford Popper the respect due to a philosopher who wrote always with a degree of clarity, precision, and directness rare in the academic world of his time, and – as O'Hear puts it – 'even rarer subsequently'. This important book constitutes an essential introduction to some of the most esteemed philosophical writing of our times.