Socrates asserts that not only can the Form of Tallness never admit of shortness, but also the tallness in us can never admit of shortness either: in the presence of shortness, tallness either withdraws or disappears completely. While Socrates can be tall with respect to one person and short with respect to another and still retain his identity, Tallness cannot admit its opposite quality, Shortness, without losing its identity.
Socrates notes that this present argument does not conflict with the Argument from Opposites. It is true that opposite things come from opposite things, but the opposite itself--meaning both opposite Forms and the opposite in us--can never become its opposite. A tall man can come to be tall out of shortness in the sense that he was short before he became tall, but the tallness itself does not come into being out of the shortness.
Next, Socrates points out that certain things in the world invariably possess a characteristic Form. For instance, snow is always cold and fire is always hot, though snow is a thing quite distinct from cold, as is fire from heat. Still, snow must always be cold, and cannot admit of heat without either withdrawing or disappearing completely--and similarly with fire. Socrates also notes the same thing with mathematics: three and oddness are two distinct things, and three must always be odd if it is to retain its nature. And while Two and Three are not opposites, they are invariably connected with Evenness and Oddness, respectively, which are opposites, and so Two can never become Three without losing its own nature.
Not only do opposite Forms not admit of one another, then, but there are also things which cannot face the approach of opposites. These things, Socrates claims, are those that compel a thing not only to admit of its own nature, but also of a Form which is an opposite. For instance, suppose three pencils are put together so that they participate in the Form of Threeness. Not only does Three compel the pencils to participate in its own nature, but it also compels them to participate in the Form of Oddness, thereby excluding the Form of Evenness. Thus, the three pencils can never become even without ceasing to be three.
It follows from this discussion that there is now an alternative explanation as to how something may come to acquire a certain quality. Before, the only explanation for a burning log's becoming hot was that it was participating in the Form of Heat. Now, Socrates suggests, we can say that the log becomes hot because of the fire, which is invariably accompanied by the Form of Heat.
Whenever a soul occupies a body, it always brings life with it. This would suggest that the soul is intimately connected with life, and so cannot admit of its opposite, death. If that which does not admit the Form of Evenness is uneven, then it follows that the soul, which does not admit of death, is undying.
Since the soul, as intimately connected with the Form of Life, cannot admit of death, it must either withdraw or disappear at the approach of death. But if the soul is undying, it cannot possibly disappear and perish. Therefore, it must simply withdraw at the approach of death. Therefore, Socrates concludes, the soul does not die with the body, but simply withdraws from it, living on, eternal and indestructible. At this point, Cebes admits that he is entirely convinced by Socrates' argument.
Plato's fourth and final argument for the immortality of the soul is probably the best and most convincing. Certainly, Plato treats it as such. It is the most direct application of the Theory of Forms to the question of the soul's immortality, and, as Plato constantly reminds us, the Theory of Forms is the most certain of all his theories.
It seems that Plato is asserting here a third metaphysical level in his theory. If we take the example of a beautiful person, not only does Plato want to say that there is the Form of Beauty and there is the beautiful person, but he also seems to say that there is the beauty present in that person which is distinct both from the person and from the Form. This beauty can come and go, and must, for instance, either withdraw or disappear at the approach of ugliness. This might lend some credence to the view of Forms as stuffs: there is a little bit of beauty in each person, which comes and goes as it pleases.
Plato also asserts here the existence of essential properties. (This idea will bear far more weight in Aristotle, but it does play an important role in this argument.) The distinction is drawn between essential properties, properties without which the thing that has them would be other than it is, and accidental properties, properties that just happen to hold of a thing, but which it could do without. So, for instance, being orange is an accidental property of fire, whereas being hot is an essential property. A fire can become blue, for instance, and still be fire, but a fire cannot be fire unless it is hot: there is no such thing as a cold fire.
Analogously, Plato suggests that life is an essential property of the soul, whereas its existence within a body is only an accidental property. So a soul can leave the body and still be a soul, but it cannot be a soul unless it is alive. Further, because it cannot possibly admit of death, the opposite of life, a soul cannot die. We should note that there is a difference between being destroyed and dying, but both of these options are ruled out for the soul. Pre-Socratic physics held that nothing could cease to be without being replaced by something else, so absolute destruction is not possible. The living soul cannot simply be destroyed, but would at the least be replaced by something else--namely, death. But since the soul's essential property rules out this possibility as well, it must live on forever.