If you haven't painted grey, Paul Cézanne once said, you are not a painter. The same could be said of philosophers: if you haven't thought grey, you are not a philosopher. Baldly stated, this might seem absurd - or at the very least, a pointless provocation. Why should philosophers think about a single colour instead of devoting themselves to ethics, metaphysics or logic? But the history of philosophy lends support to Sloterdijk's intuition. What colour are the shadows in Plato's allegory of the cave? Did not Hegel write, in the Preface to Elements of the Philosophy of Right , that 'When philosophy paints its grey in grey, one form of life has become old, and by means of grey, it cannot be rejuvenated but only known'? And doesn't Heidegger's being-in-the-world imply existing in a diffuse shade of grey? In this exceptionally original book, Peter Sloterdijk follows the grey thread through the history of philosophy, art and culture. Storms, sea mist and cloud-soaked mountains clad a chiaroscuro narrative which recognizes the power of grey as a metaphor for public opinion and a signifier for political and moral ambiguity.