This chapter examines Spinoza’s claim in TP, Chapter 6 that a multitude unites to form a political body prompted not by reason but by some common affect: fear, hope, or desire to avenge a common injury. This chapter examines the possibility, realisability, and legitimacy of such a paradoxical and problematic form of unity. It demonstrates the originality of Spinoza’s thesis, which has not been recognised by commentators. It examines the problems a foundation of revenge involves, such as durability, susceptibility to violence, and legitimacy. It concludes with a definition of the “correct use” of revenge by distinguishing carefully, as Spinoza does, a passion for revenge that derives from desiderium rather than from cupiditas. It illuminates the precise kind of vindictive affect that can ground a multitude’s agreement, and thus sovereign law and common justice.