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.

In Plato’s dialogues, Socrates calls things like justice, piety, and largeness “forms.” In several of these dialogues, he makes clear that forms are very different from familiar objects like tables and trees. Why, exactly, does he think that they differ and how are they supposed to do so? This chapter argues that in the Phaedo Socrates does not assume that they are different, but rather, over five stages of the dialogue, provides an account of how and why they do so. To fully understand the claims made in the first stage, one must look to the next stage, and so on until the final stage. Socrates' ultimate reason for distinguishing forms from ordinary objects does not depend on our intuitions about things like justice and largeness, nor on the distinction between universals and particulars. Ultimately, forms cannot be ordinary objects because the form of f-ness must cause every f-thing to be f, but no ordinary object could serve as such a cause. They cannot do so because they have multiple parts and are receptive of opposites; by contrast, the form of f-ness must be simple and unchanging, since it causes every f-thing to be f.

In dialogues ranging from the Symposium to the Timaeus, Plato appears to propose that the philosopher’s grasp of the forms may confer immortality upon him.  Whatever can Plato mean in making such a claim?  What does he take immortality to consist in, such that it could constitute a reward for philosophical enlightenment?  And how is this proposal compatible with Plato’s insistence throughout his corpus that all soul, not just philosophical soul, is immortal?  In this chapter, I pursue these questions by applying the distinction between general and earned immortality to the Phaedo and the Symposium.  I argue that, while Plato attributes general immortality to all souls in the Phaedo, he proposes in the Affinity Argument that the philosopher’s soul can achieve earned immortality through contemplating forms.  It is this form of immortality that Plato claims is unavailable to humankind in the flux passage of the Symposium.  At the same time, in the ascent passage, he holds out the possibility – albeit with significant reservations – that the philosopher’s soul may transcend its humanity and achieve earned immortality through its communion with the forms.