This chapter situates the problem of the double in, primarily, Plato and Kant—supplemented by aspects of Deleuze and Derrida—using them to represent the necessity of the work of the double in the history of philosophy. I set up my approach with reference to doubles in the philosophy of Sartre, and then focus here on splits and divides in Plato, both in the divided line and the allegory of the cave, but most distinctively in his attempt to define the identity of the philosopher in contrast with the elusive sophist. Finally, I turn to the sharp divisions that for Kant characterize his Copernican Revolution.