1.) P-form. Whatever you understand when you understand something.
2.) The soul understands itself. Therefore the soul itself is a P-form. While other animals and plants have souls, only the soul of the person is, in addition, a p-form.
To update, “consciousness” can be replaced for “soul”. Clarification: the self-knowledge in question is not a knowledge of what soul/consciousness is, but initially only that it is. An actual theory, as opposed to a sign of it or a record of it, is an actual way consciousness is.
3.) But no P-form can be other than itself. Equal things can become unequal, but equality cannot be; an even number of stars might become an odd number, but an even number will never be odd.
4.) No p-form soul will cease to be itself.
5.) But all that ceases to exist, ceases to be itself.
6.) No soul ceases to exist.
Commentary: The basic inference is from self-reflection to self-subsistence, by noticing the common feature between formal abstractions and the concrete human soul (both are immediate objects of understanding). Plato seems to leave out the crucial element of his argument that the human soul is a p-form while this is not true of all souls, although many of the problems of the Phaedo are resolved by doing so.
Phenomenologically, the self-reflection of soul is why we relate to both the world and our beliefs as true. We see truth because we can compare our thoughts to the world. Lacking this, science is impossible, and sciences are demonstratively impossible for non-persons (which is in many ways a pity. I kinda wish we could get physical theories from animals with other sense powers than ourselves.)