From Chapter 3 "Plato's Near Miss: The Soul as Self-Moved," in Hartshorne's Insights and Oversights book.
How did Plato conceive soul or psychical process? After exploring the logically possible sorts of motion or change, he finds one of them unique to soul, that of self-change, together with the power to change others. Souls initiate change as well as transmit it to others. In contrast bodes merely receive and transmit change. Souls create processes, other entities merely constitute them…What does Plato mean by self-moved or self changed?...Whatever self-motion may be for Plato, it cannot be the only power of soul or psychical process if it is to provide the explanation of change generally. We need to ask, What gives psychical or self-changing process the additional power to change others?...If soul can explain reality, it must a threefold power: to initiate change, receive it from, and transmit it to, others. Bodies must have at least the second and third of these powers…How close or not close Plato came to solving the problem dualism is shown by the following, from The Laws X (897, 897):The self-moved is…that which has the name soul.The self moved…is the source of change and motion in all things.[The movements of soul are] will, deliberation, joy, sorrow, confidence, fear, hatred, love, and other primary movements which again receive the secondary movements of corporeal substances and guide all things.Here matter is contrasted with mind in the negative, as that which is without self-movement, but in the positive as that which is corporeal….Leibniz argued that whatever is positive in spatiality is attributable to mind. This positive meaning is coexistence. And few philosophers have denied that minds coexist. Thus matter is “not-mind” only by negation. And negation always implies a positive grounding.It is remarkable that Plato seems almost to think that mind “receives” motions only from bodies, not from others minds. But clearly souls are influenced constantly by other souls, at the very least by the supreme or divine soul…Plato’s attempt in the Phaedo to derive personal immortality from the primacy of soul as self-changing and the key to all change is, so far as I can see, a confusion between soul as such and this or that individual soul. Soul as such is immortal in the sense in which change is unchangeably there. Reality cannot change but from a changing to an unchanging reality; hence, not from a besouled to merely unbesouled reality…That souls are moved from without is implied by the Platonic doctrine of Eros, as expounded by Diotima in the Symposium: Love (in this sense) implies deficiency, which one seeks to overcome by admiration and longing directed to the Absolute Beauty, the good that is without deficiency. And what is this beauty?...Is absolute beauty the concrete union of all possible values or “perfections,” all fully actualized? This idea conflicts with the truth (perhaps first clearly stated by Leibniz, who, however failed to draw the implied theological conclusion from it) that there are “incompossibles.” Not all possible values could be coactualized. Nor will it do to argue, as Leibniz did, that in the divine perfection there are only positive values and that these cannot conflict….On the contrary, positive values can and do conflict, as Kant pointed out….There are contraries as well as contradictories! Red-here-now conflicts with green-here-now, and the one is as positive as the other….The expression Absolute Beauty has never been given a lucid explication, unless in the formula of Leibniz, maximal unity in maximal variety, or as he put it in his rationalistic way, “maximal consequences from minimal premises.” But what does “maximal” here connote? It cannot be all possible variety, for that could not be actualized? What short of that? Leibniz could not tell us…Aesthetic value is logically incapable of an unsurpassable maximum. There can be a factual maximum, namely all actual variety ideally integrated. But any actual variety is logically capable of being surpassed…Perhaps another reason why Plato almost omits capacity to be moved by another from soul’s nature is that he thinks it would imply mortality; moveable by another, then destructible by another (Phaedrus 246). However, capacity to be moved by another implies mortality only with certain qualifications. An individual that can be influenced by its environment must, to maintain its integrity, “adapt” to that environment. It must be able to find a response to stimuli compatible both with the forces and with its own already formed character. In this way morality is implied: The environment may present forces to which the individual cannot adapt at all, in which case it dies. But suppose an individual so perfectly adaptable, that failure of adaptation is excluded. In other words, suppose infallible powers of adaptation to (or control of) the environment, the union between the two powers constituting an infallible capacity to survive environmental change. Just this is how I read the account of the “world soul” in the Timaeus….