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.