Looking back across the history of Western philosophy, we find a long line of tradition that emphasizes the unity and simplicity of the minds of epistemic and moral agents. For Aquinas, for instance, the mind’s ability to engage in higher-level cognition crucially depended on its simplicity: if it was to cognize the universal natures of things, it had to be simple and incorporeal rather than material, complex, and therefore limited in nature. 1 Moreover, many held that the identity of particular agents in and through time required an underlying simple substance. Only a simple substance, Plato for example argued in the Phaedo, is guaranteed to be stable, since any change would imply a change of parts. 2 Relatedly, moral agency—in particular once combined with the Christian promise of just reward and punishment after death—was often thought to call for a simple soul that would provide a fixed locus for the attribution of responsibility even after an agent’s bands of memory had become frayed.