There is a great deal of debate as to what Forms exist, whether they are just abstract conceptsfor example, if there can be a Form of Cat, or a Form of Chair that all cats or chairs participate in. The Theory of Forms is probably Plato's most significant and lasting contribution to philosophy, and it has taken many forms since Plato's day. But despite the importance of Forms to Plato's philosophy, there is mystifyingly little discussion of the Theory itself in Plato's writings. For instance, in the Phaedo, we stumble upon the theory at 65d, where Socrates asks Simmias if he recognizes such things as justice, beauty, and goodness in themselves. Not only does Socrates not give any technical background or argument for their existence, but Simmias unreservedly agrees with Socrates' assertion. (In fact, the only dialogue in which the Theory of Forms itself is the subject of a philosophical debate is the Parmenides, and in this dialogue, the Theory is subjected to a devastating attack.) In the Phaedo, we are just offered the Theory in passing and are expected to accept it as given.
We might be inclined to question the ease with which the soul is introduced to the discussion here. Much like the Theory of Forms, the idea that death is a separation between soul and body is simply accepted unquestioningly by Simmias, at 64c. However, Socrates does not bring in much metaphysical baggage with this assertion. In particular, as we shall see, he has not committed Simmias to the claim that the soul coheres as a distinct entity after death. All that has been agreed upon is that the life-giving element, which we call the soul, leaves the body at death, which is why the body ceases to be alive.
Perhaps more interesting and controversial is Socrates' association of reason with the soul and the senses with the body, coupled with the assertion that the senses are fallible and that all true knowledge comes by way of reason. This Simmias agrees to at 65b-c. Questions regarding the nature of knowledge and how we come to know form the major branch of philosophy known as epistemology. (Plato's main treatment of epistemology is found in the Theaetetus, and there is also a significant discussion of knowledge in the Meno.) There is a great deal of debate, even today, as to the respective roles played by reason and the senses in human knowledge. Empiricist philosophers, such as Locke, Hume, or Bertrand Russell, place a greater emphasis on the senses, while rationalist philosophers, such as Descartes, Spinoza, or Leibniz, follow Plato's lead in giving reason a more prominent place.
One final item worthy of note concerns the analogies Socrates uses in comparing our relationship with the gods, and the soul's relationship with the body. Our relationship with the gods is usually portrayed as similar to the relationship between slave and master. (Notice that this is an implicit endorsement of the slavery that maintained Athenian society: it is better that we obey the gods as our masters and accept our role as their slaves than have the arrogance to think that we could do better on our own.) When discussing the soul, we are often given the image of the soul imprisoned by the body. Thus, death is a kind of liberation from this prison. We might want to ask ourselves, if the gods are such good masters, why have they imprisoned us within these imperfect bodies, torturing and restricting the freedom of our souls?