Castoriadis's radical ontology of indeterminacy postulates a third term (or rather, an indeterminable continuum of terms) between the complete determinacy of the traditional conception of being and the absolute indeterminacy of the traditional conception of nothingness. Castoriadis himself made considerable efforts to demonstrate how ontological conceptions which equate being with determinacy fail to grasp the reality of being in all ontological regions and contexts. He did somewhat less in regard to the opposite pole of the ontological dichotomy, the identification of indeterminacy with nothingness, though he certainly recognized this identification as equally suspect. This article examines the use and interpretation of the concept of nothingness in two of the most important and exemplary employments of this concept in twentieth century philosophy. Nothingness is a key concept for both Heidegger and Sartre, however they may differ in regards to its meaning and significance. In both, the concept of nothingness disguises indeterminate modes of being which both perceive but which they refuse to accord the status of being, preferring instead the stark and absolute opposition between being, which continues to be understood in traditional terms as determinacy, and nothingness, a catch-all category which absorbs and homogenizes anything falling short of the absolute determinacy predicated of true being.