When Socrates first introduces the notion of Forms, the people he’s talking to accept the existence of Forms without further debate, leaving us to ponder on our own why we should accept them. Further, we get no clear sense of how many kinds of Forms there are or how exactly they interact with their manifestations in the world of experience. Discussions of Forms in Plato’s other dialogues normally focus on abstract ideals such as the Form of Beauty or the Form of Justice. In the Phaedo, however, Socrates mentions a Form of Duality, a Form of Equality, and even, under certain interpretations, a Form of Fire and a Form of Snow. Not only do these examples leave us with no defined limit of what kinds of Forms there are, but they also raise a series of problems that do not arise with abstract ideals. We might ask, for example, how the Form of Equality can itself be a perfect paradigm of everything that is equal when equality is a relative term, meaning that nothing can be equal in and of itself but can only be equal in relation to other things.
Each of the four arguments for the immortality of the soul does different work in the dialogue, even if they all aim at proving the same thing. The Argument from Opposites absorbs a line of thinking that was popular among earlier philosophers such as Heraclitus and Pythagoras. By following their lead in seeing the world as being divided into opposites, Plato presents an initial argument that would be sympathetic to his contemporaries. The Theory of Recollection introduces the idea of Forms and, in associating knowledge with the immortal soul, suggests that the soul that survives death is not just an empty life force but includes the intellect. The Argument from Affinity makes explicit the distinction between the soul and the body. By asserting that different fates await different souls depending on how purified the souls are of the needs of the body, Plato endorses the philosophical life. The final argument based on Forms is the only one Plato deems truly definitive, refuting the doubts of Simmias and Cebes.
The distinction Plato draws between the body and the soul was revolutionary in his day and is one of the earliest forms of what we now refer to as “mind–body dualism.” Dualism is the idea that mind (or soul) and body are distinct substances with distinctive natures. Plato goes so far as to suggest they are opposites, placing the soul and body in two opposing categories in the Argument from Affinity. He identifies the self with the soul, suggesting that we have no reason to fear death since it is only our body and not our self that will perish. This identification of self with soul raises some question as to what counts as our “self.” Our thinking is largely informed and inspired by what we see, hear, and sense, and our senses are a part of our body that will not survive death. We might doubt whether we can shed the body, and all the influences we draw from it, and become a soul of pure intellect that we can readily identify with the “self” that we think we have.