One particular feature of Locke's Essay Concerning Human Understanding - the suggestion that God could add to matter the power of thought - stimulated an extensive debate in Britain between immaterialists (those who defended two substances, mind and matter) and materialists (those who considered matter to be self-active). That debate was also transmitted to the Continent, especially to France, where Locke's suggestion about thinking matter was given prominence by Voltaire. His defence of the suggestion was in turn attacked by a number of writers, thereby implicating Locke in the growth of materialism in France. By the middle of the eighteenth century, Locke's `famous hypothesis' had become the centre of many attacks, mainly by followers of Malebranche. This book tells for the first time the long and complex story of the involvement of Locke's suggestion in the growth of French materialism. There is a discussion of the `affaire de Prades', in which Locke's name was linked with a censored thesis at the Faculty of Theology in Paris. The similarities and differences between English `thinking matter' and the French `matière pensante' of the philosophes are discussed in the last chapter.