This argument (typically called the “affinity argument”) is central to the structure of the Phaedo, setting up much of the remainder of the dialogue. Moreover, it develops the dialogue’s most detailed account of the forms and of ordinary objects, and it argues for an innovative account of the nature of the soul, which is relied upon in Socrates’ ethical account in the next section. Despite this, the argument has received very little scholarly attention, supposedly because scholars widely view it as an especially bad argument. This chapter shows that the argument is much more precise and stronger than has been appreciated. In doing so, it argues that Socrates describes here a new, fundamental feature of the forms: they are simple in a way that makes them partless – in strong contrast to ordinary objects, whose complex structure allows them to have opposing features at the same time.