Between them, the movements of Logical Positivism and Existentialism have dominated philosophy in Europe for much of the last century. This reissue of Copleston's acclaimed work provides a detailed and objective introduction to these two highly controversial areas of recent thought. Originally written in 1956 and revised in 1972, this book explores the work of many of the most important thinkers of the 20th century, including Ayer, Wittgenstein, Heidegger, Sartre and Camus.