The chapter considers the concept of ‘existence’ as it is variously applied in Locke to the objects of sensation (the ‘real existence’ of things) and to the objects of reflection (the ‘fleeting existence’ of ideas). It shows that Locke, in order to construct his own ontology and typology of simple ideas and modes, is both using and subverting the Cartesian ontology of substance and modes. Ideas, as ‘immediate objects of perception’, exist in the mind, but not substantially. This in turn sheds light on the differences between Locke’s and Descartes’s doctrines of ideas, especially on the question of ‘objective reality’, which played a strategic part in the Third Meditation, as well as in the debate between Arnauld and Malebranche.