If the soul is one, and immortal, it cannot alter. For if it were, say, to move closer to or further from some afterlife, that would be an alteration. Because the soul is one, any alteration must be complete; that is, there is no part to the soul, so an alteration necessarily alters it entirely (the soul is atomic). And if the soul defines identity, any alteration would produce a new person. And a new person cannot be held responsible for the actions of another, and so cannot move closer to or further from some afterlife. Thus, the soul cannot alter.