Echecrates presses Phaedo of Elis to give his account of Socrates’ death. Socrates had been condemned to commit suicide by drinking hemlock, and a number of his friends and fellow philosophers had gathered to spend his last hours with him. Phaedo explains that among those present with him were Crito and two Pythagorean philosophers, Simmias and Cebes.

In Phaedo’s account, Socrates explains to his friends that a true philosopher should look forward to death. The purpose of the philosophical life is to free the soul from the needs of the body. Since the moment of death is the final separation of soul and body, a philosopher should see it as the realization of his aim. Unlike the body, the soul is immortal, so it will survive death.

Socrates provides four arguments for believing the soul is immortal.

He bases the first, known as the Argument from Opposites, on the observation that everything comes to be from out of its opposite. For example, a tall man can become tall only if he was short previously. Since life and death are opposites, we can reason analogously that, just as the living become dead, so the dead must become living. Life and death are in a perpetual cycle such that death cannot be a permanent end.

The second argument, known as the Theory of Recollection, asserts that learning is essentially an act of recollecting things we knew before we were born but then forgot. True knowledge, argues Socrates, is knowledge of the eternal and unchanging Forms that underlie perceptible reality. For example, we are able to perceive that two sticks are equal in length but unequal in width only because we have an innate understanding of the Form of Equality. That is, we have an innate understanding of what it means for something to be equal even though no two things we encounter in experience are themselves perfectly equal. Since we can grasp this Form of Equality even though we never encounter it in experience, our grasping of it must be a recollection of immortal knowledge we had and forgot prior to birth. This argument implies that the soul must have existed prior to birth, which in turn implies that the soul’s life extends beyond that of the body’s.

The third argument, known as the Argument from Affinity, distinguishes between those things that are immaterial, invisible, and immortal, and those things that are material, visible, and perishable. The soul belongs to the former category and the body to the latter. The soul, then, is immortal, although this immortality may take very different forms. A soul that is not properly detached from the body will become a ghost that will long to return to the flesh, while the philosopher’s detached soul will dwell free in the heavens.

Both Simmias and Cebes raise objections to these arguments. Simmias suggests that the soul may be immaterial and invisible in the same way as the attunement of an instrument. The attunement of the instrument can exist only as long as the instrument itself does. Cebes accepts that the soul may survive death, but he suggests that Socrates has proved only that the soul lives longer than the body, not that it is immortal.

Socrates responds to Simmias first, pointing out that his objection conflicts with the Theory of Recollection. The soul is not like the attunement of an instrument because the soul existed before the body did.

His answer to Cebes involves a lengthy discussion that culminates in his fourth argument, based on the Theory of Forms. A Form, unlike qualities in this world, is perfectly itself and does not admit its opposite. For example, the Form of Beauty does not possess any ugliness at all. In contrast, a beautiful person might be beautiful compared to other people but would not seem beautiful compared to a god and thus is not perfectly beautiful. The Form of Beauty, on the other hand, is always and absolutely beautiful.

The soul is what animates us: we are alive because we have a soul. That concept suggests that the soul is intimately connected to the Form of Life. Since the Form of Life does not in any way include its opposite—death—the soul cannot in any way be tainted by death. Thus, Socrates concludes, the soul must be immortal.

Socrates illustrates his conception of the soul by means of a compelling myth that describes the earth we know as a poor shadow of the “true earth” above us in the heavens. Then he has a bath, says his last good-byes, drinks the poisonous hemlock, and dies peacefully.

The Theory of Forms is the most important philosophical aspect of the Phaedo and central to Plato’s thought in general. Inspired perhaps by the perfect clarity and permanence of mathematics, Plato doubts that the world of our experience, where nothing is perfect or permanent, can really be all there is. Even though all the instances we find of justice and beauty in this world are flawed in some way, we still instinctively have a sense of what true justice and true beauty are. Plato’s theory explains that above the unsatisfying world of our experience there is a world that contains the Form of Justice, the Form of Beauty, and other Forms that similarly embody the perfect expression of these ideals. Any beauty or justice we find in this world has beauty or justice only to the extent that it partakes in these Forms. The beauty and justice we find in this world are like shadows cast from above that give us some indication of the nature of the more real world of Forms.

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.