Cebes brings up Socrates' Theory of Recollection, which claims that all learning is recollection. Simmias cannot quite remember the proof of that theory, and asks for an explanation. Socrates begins by pointing out that we can be reminded of one thing by being made conscious of another thing. For instance, if one sees a lyre or an article of clothing that belongs to a beloved, one will immediately be reminded of the person whose lyre or clothing it is.
Socrates now re-introduces the Theory of Forms, making Simmias agree that there is such a thing as Equality itself--something that is independent of any particular case of equality such as equal sticks or equal stones. We know this Form of Equality, because it comes into our minds every time we see instances of equal objects. However, Socrates points out, equal stones or equal sticks may look equal from one point of view and unequal from another. Nonetheless, we would never be tempted to suggest that Equality itself is unequal. Therefore, the sticks or stones that are equal cannot be the same thing as Equality, since they can sometimes be unequal, and Equality itself never can be. If the equal things are different from Equality and yet can bring Equality into our minds, they must somehow remind us of the Form of Equality. We are aware that the sticks or stones fall short of being perfectly equal, but to be aware that they fall short, we must already have an idea of what it means to be perfectly equal; that is, we must already know the Form of Equality.
We become aware of the equal sticks and stones through our senses, and similarly sense their deficiency with respect to true Equality. There are no instances of perfect equality in the sensible world, and yet we have had this notion of Equality for as long as we have been alive. Socrates infers that we cannot have come to learn of Equality through our senses, but that we obtained our knowledge of it before our birth. And if this holds true of Equality, it should hold true of all the other Forms as well. It would seem that we lose knowledge of these Forms at birth, and it is through a process of learning that we come to recollect them and know them again. This is why Socrates claims that all learning is recollection.
Next, Socrates presents an alternative explanation of the same thing. Someone who truly knows a subject ought to be able to explain it to others, yet most people cannot explain the things that Socrates has been explaining to Simmias. If they cannot explain these things, but can be brought to recollect them to such a point that they might be able to explain them, they must have acquired knowledge of them in some past life that they forgot at the moment of birth.
Simmias and Cebes agree that Socrates has shown that the soul existed before birth, but they remain unconvinced that the soul coheres after death. Socrates remarks that this has already been proved, if we combine the Theory of Recollection with the Argument from Opposites. The Theory of Recollection shows that the soul existed before birth, and the Argument from Opposites shows that it must have been born from out of death. Bearing in mind that the soul has to be re-born after it dies, Simmias and Cebes are forced to acknowledge that it must continue to exist after death.
The Theory of Recollection is laid out in more detail in Plato's Meno, and the discussion in the Phaedo alludes to, and seems to assume prior knowledge of, this earlier discussion. The Phaedo and the Meno are consistent, though, and the presentation of the theory in each dialogue can stand on its own.
This theory gives further insight into the epistemology of Plato's middle period. First, it is worth noting that empirical knowledge--knowledge we gain from experience--does not count as knowledge for Plato. For instance, an empirical fact such as "Sparta won the Peloponnesian War" cannot be recollected nor brought into the mind by means of questioning. It would also be absurd to say that we have had this knowledge in our souls since before we were born: Socrates was born before the Peloponnesian War, so how could he have had this knowledge back then? True knowledge for Plato is only knowledge of the Forms. If we know Equality and Beauty and Justice and all the rest, then our understanding of the sensible world will flow from these Forms. For instance, I can know that two sticks are almost, but not quite exactly, equal because I know the Form of Equality.
Plato also wants to discuss two different kinds of knowledge. It seems that on one hand, we all have intrinsic knowledge of the Forms, which is why we need only recollect them to bring them into our minds. On the other hand, however, this knowledge remains somehow sub-conscious until it is brought out through recollection. People who have conscious knowledge of something are able to give an account or explanation of what they know, and teach it to others.
This section of the text, like much of the Phaedo, is quite problematic. Three main problems crop up here, which we shall discuss in turn. First, there is an objection to the Theory of Recollection. The second and third problems focus on Socrates' discussion of the Form of Equality, which raises some questions and problems for the Theory of Forms itself.
Socrates seems mostly intent on showing that we came to know what we now know before birth. However, he never gives any explanation of when before birth we acquired this knowledge, nor how. If we came into this life knowing what Equality is, when did we first come to know Equality? If no true knowledge comes from experience but is all innately given, experience from a previous life could not have given us knowledge of Equality either. Perhaps Plato would suggest that when our soul is first created, and has its first life, it is created with a knowledge of Forms. But this raises another question that Plato does not answer: when and how does the cycle of birth and death of the soul begin? And if it has a beginning, why can't it have an end?
Our other two problems concerns the Theory of Forms. Socrates asserts that the Form of Equality is different from equal objects in that equal objects can sometimes be seen as equal, sometimes unequal. Equality itself, on the other hand, is always equal. Plato seems to propose this as a general rule for Forms: the Form of Equality is equal, the Form of Beauty is beautiful, the Form of Justice is just, and so on.
The first question this raises is what is meant by "is" in this case. It could be: (1) the "is" of identity, such as "Socrates is the teacher of Plato" where both sides of the "is" mean the same thing. (2) The "is" of predication, where the predicate is an essential characteristic, as in "Socrates is a human being." Under this interpretation, "The Form of Equality is equal" would mean that the Form of Equality is essentially and above all, equal. (3) The "is" of extension, where the Form of Equality is equivalent to the set of all particular objects in the world that participate in this form.
The second question asks what Plato intends by a Form of Equality. We normally think of equality as a relation that holds between two things. Two equal sticks are equal not through any intrinsic properties they have, but through their relation to one another. Plato, in advancing Equality as a Form, suggests that Equality is a property in itself that can hold of objects in themselves. Thus, in the case of our two equal sticks, each stick participates in the Form of Equality independent of the other. However, we wouldn't normally think of a single stick on its own of being able to have the property of equality. In saying that Equality is a property that is intrinsic to objects rather than holding between them, Plato makes himself vulnerable to a whole series of objections.