Gilles Deleuze once famously stated that Spinoza’s remark that ”we do not know beforehand what a body can do” should be read as a cri de guerre against a philosophical tradition that ever since antiquity has devalued the role of the body in the formation of knowledge. This paper further investigates the role of the body in Spinoza’s Ethics by examining what I shall call the becoming-active of the body. Commentators often interpret activity as pertaining to the formation of rational thought through the so-called ”common notions” in the Ethics. However, in doing so, they often neglect the central role played by the body in the process of becoming active. Thinking, for Spinoza, is always ”in the midst of things”, and as such it must be interpreted according to the basic affective relationality of the body. The act of thought; the activity of thinking, must be understood as an expression of a more fundamental activity of the body if we are to remain true to the implications of Spinoza’s monistic ontology. The goal of this paper, then, is to show how the becoming-active of the body is a necessary correlate to the activity of the soul in the Ethics. By demonstrating this, I will show in the concluding sections of my investigation, how the role played by the body and affectivity in Spinoza’s philosophy forces us to reconsider not only ethics, but the activity of thinking in general.