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 GC II 2, Aristotle discovers the contrarieties that constitute the principles of all perceptible bodies. They are the hot and the cold, the dry and the moist because they are the basic tangible contrarieties that interact as active and passive powers in a way that can result in a new affection. In GC II 2, Aristotle also attempts a reduction of all other tangible contrarieties to the basic four. I argue that the active/passive relation does not hold only between the two basic pairs of contrarieties but also within each pair. I compare the reduction of other contraries to the basic four with what Aristotle says in Meteor. IV. Despite minor differences and the wider scope of analysis in the latter work, Aristotle’s admirable effort to reduce the rich variety of tangible properties to a limited number of contraries and elements is the same.