The immediate infinite mode of extension is one of the more mysterious elements of Spinoza’s ontology. Despite its importance to Spinoza’s metaphysical system, Spinoza tells us very little about this mode. In the effort to make some progress on this question, I examine three prominent hypotheses about this mode’s identity: the Force Interpretation, the Nomic Interpretation, and the Kinetic Interpretation. I argue, first, that both the Force Interpretation and the Nomic Interpretation are subject to a number of serious objections. Second, I argue that in spite of scholars’ claims to the contrary, the Kinetic Interpretation is a live option.