The founders of sociomateriality argue that the difficulty of maintaining distinctions between material and social entities, increasingly encountered empirically, also necessitates a commitment to inseparability at the ontological level. Yet reservations have been voiced that this proposal is contrary to experience, incoherent, and of little practical use. While not denying the phenomenon, the critics insist that the ontological commitment to separated entities must be maintained. We show that Heidegger’s work in Being and Time (1927, 1962) provides a careful exploration of ontological inseparability that reveals it to be plausible, coherent, and useful. Although his terminology is alien, Heidegger’s analysis proceeds from everyday experiences to provide a plausible account of ontological inseparability. His work provides a coherent ontology that treats inseparability as fundamental, but also shows that our familiar dualist experience of the world, far from justifying ontological separation, is actually derivative. We demonstrate the usefulness of Heidegger’s concepts with a case. Instead of proceeding from a priori separated categories meaningful to the researcher, Heidegger’s concepts allow investigation of what IT "is" when involved in particular user worlds. We conclude that IS must engage with the "being of IT" if it wants to adequately deal with the increasing ubiquity of IT in practice.