In this paper, I explore Spinoza’s conception of the relation between substance and mode through an analysis of the notions of self-causation, immanent causation and power. I argue that Spinoza’s theory of causality constitutes an attempt to develop a relational ontology of power where all intrinsic denominations are grounded in extrinsic ones. In opposition to a current reading according to which Spinoza’s substance is similar to a huge monad in which things inhere in the same way as predicates inhere in a logical subject, I argue that the ontology of immanence implies a radical exteriorization of being: for a finite mode, “to be” is to exteriorize itself as a cause, i.e. to act. As for substance, it is nothing but causality or action as such, an absolute form of exteriority. Hence, when Spinoza maintains that all things are “in God” (in Deo), the preposition does not signify the interiorization “in a subject” (in subjecto), but rather an exteriorization “in action” (in actione).