A third argument for the continued existence of the soul after death is based on an analogy between the soul and the Forms. The Forms are not the kind of thing that would cease to exist, because they are not composed of parts. The soul resembles the Forms in various respects, such as being invisible and metaphysically superior, so it probably resembles the form with respect to simplicity as well.
1. If the soul ceases to exist, it must be because it has decomposed
2. The Forms are simple and incapable of decomposition
3. The soul resembles the Forms in its simplicity
4. So, the soul is incapable of decomposition
5. So, the soul cannot cease to exist
In the text, we are told initially that it is "likely" that what is not composite will not split up. Plato then shifts the emphasis to the unchanging character of the Forms, eventually declaring them to be incapable of change. The argument could be interpreted as saying that the soul resembles the Forms in some other respects, and by analogy it should be incapable of change as well. But then this would not explain why composition and simplicity are discussed in the first place. I have attempted to integrate the considerations of simplicity with the changeless character of the form. The resulting argument then resembles one that had some currency in the seventeenth and eighteenth centuries, though without the reference to the forms. Simplicity was attributed to the soul on other grounds. Immanuel Kant tried to prove that we couldn't know that the soul is simple (Critique of Pure Reason, Paralogisms of Pure Reason). At any rate, step 3 of the argument would have to be established by analogy, and so the argument is not as strong as Plato might like it to be. And it need hardly be added that the existence of the Forms and their divine character is highly controversial.
