In a developmental study, one proposes an explanation for an observed change. Thus, a developmental study requires that one establish first that there really is a change.
Bobonich explains a change in terms of a change, and so he needs to establish that there really are two changes in Plato's thought. And in each case there should be at least a logical distinction between one's establishing the change, and one's giving the explanation for that change, otherwise one risks conflating one's theory with the evidence for one's theory.
What do I mean in saying Bobonich explains a change in terms of a change?
Change 1: Plato in the Phaedo and Republic holds that non-philosophers cannot have virtue or happiness; Plato in the Laws holds that non-philosophers can be virtuous and happy.
Change 2: Plato in the Republic holds a tripartite theory of the soul; Plato in the Laws rejects the tripartite theory of the soul.
Change 2 accounts for Change 1.
So far, I have largely been investigating whether one can establish Change 1. To establish a true change, one needs to show that the precise sense in which something is affirmed in one dialogue is denied in another. It seemed upon investigation that Bobonich's interpretation of the preludes in the Laws, on which his view is based, was not well unsupported; it seemed that Bobonich's interpretation of the Phaedo and Republic was not suitably qualified and unsupported. But, when suitable qualifications are introduced, then it's not clear that there is any noteworthy change at all. The qualified way in which the Laws recognizes virtue and happiness in non-philosophers looks not unlike the qualified way in which the Phaedo and Republic allow for virtue and happiness in non-philosophers. Any differences that remain seem to fall well within those that could easily be attributed to a shift in focus and emphasis. But if there is no change, there is no need for an explanation of that change.
But we might investigate Change 2 for its own sake, prescinding from whether it is an explanans for Change 1.
So what evidence is there, that Plato rejects the tripartite theory of the soul in the Laws? One might say: What evidence is there, that he rejects it in his (presumed) late dialogues taken as a group?-- since presumably, in the absence of further evidence, we should regard what he held in one late dialogue as what he held in all late dialogues, and it's a particular feature of Plato's Utopia Recast that it attempts to understand the philosophical viewpoint underlying the Laws precisely as an expression of philosophical views worked out in other late dialogues.
But what seems to be a big problem ('a fatal stumbling block', according to Lorenz in his review), is that Plato affirms the tripartite theory of the soul in the Timaeus (see 69c-70e), presumably a late dialogue.
Bobonich does have a response--which I'll give in a subsequent post.