‘“Dare to know!” – that is the motto of enlightenment’, wrote Kant in 1784, in his famous essay ‘What is Enlightenment?’ Clerical authority, the veracity of the Bible, miracles, magic, satanic possession, monarchy, aristocracy, hierarchy, slavery, women’s subordination to men – all were challenged. Reason, which had immemorially been subordinated to ecclesiastical authority, but which Descartes had called God’s voice speaking in each of us, became, over the period of the Enlightenment, something both more subjective and more objective – the individual’s working out of the truth, without the intervention of Church or State, in a way that would nonetheless secure universal convergence. There was a sense of transcending not only the constraints of superstition but of nationality, perhaps even of class, gender and race, in the avid pursuit of reasoned truth. ‘My dear David, you belong to all nations,’ declared Diderot to David Hume, ‘and … I flatter myself that I am, like you, a citizen of the great city of the world.’

Usually books on the Enlightenment brim with zest and triumphalism – Paul Hazard and Roy Porter delight in the era’s pursuit of happiness, Peter Gay in its enhancing of autonomy and self-reliance, Isaiah Berlin in its confidence that human omniscience can be achieved. Radical Enlightenment has none of this exuberance.