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.