The Introduction to Sartre’s Being and Nothingness (B&N) contains a condensed, cryptic argument – the ‘ontological proof’ – that is meant to establish a position ‘beyond realism and idealism’. Despite its role in establishing the fundamental ontological distinction of B&N – the distinction between being-for-itself and being-in-itself – the ontological proof has received very little scholarly attention. My goal is to fill this lacuna. I begin by clarifying the idealist position Sartre attacks in the Introduction to B&N: Husserl’s idealism as (...) interpreted by Aron Gurwitsch and Roman Ingarden. I then propose a new interpretation of the ontological proof that gives a central role to the transparency of experience. On this interpretation, Sartre argues from the ‘emptiness’ of consciousness – its purely relational nature – to the existence of mind-independent substance (being-in-itself). In assessing the strengths and weaknesses of Sartre’s case against idealism, I put him in critical dialogue with contemporary work in metaphysics and the philosophy of mind. (shrink)