After the many symmetries (subject-object, truth-falsity, etc.) that science and technology studies (STS) has explored, one symmetry seems unaddressed: the symmetry of explaining non-existence from the same kind of causes as existence. A social-constructivist perspective that articulates the work required to bring about existence cannot uncritically take for granted that the absence of such work simply explains non-existence. Rather, some non-existences are just as much “made,” and these non-existences “could have been otherwise.” In this article, I attempt to systematize this construction of non-existence under the notion of stifling. I first explore how the production of forms of non-existence has been thematized in scholarship in STS. I take non-things, non-knowledge, and non-people as the main classes to organize the arguments. I then propose a systematic approach to the ontological work performed through stifling and, notably, the consequences for what kind of futures this renders thinkable and unthinkable. Stifling then emerges as an important dynamic through which modal power is exercised and an indispensable element of ontological politics.