Hendrik Lorenz, 'Plato on the Soul' in The Oxford Handbook to Plato, p. 243 ff.

Lorenz claims that the tripartite soul is Plato's major contribution to psychology.

In the Gorgias and Protagoras, we see one version of the soul and human motivation.

In the Phaedo, we get a more advanced picture.

In the Republic, we get a good argument for Plato's most considered view.

Note that Lorenz thinks that the argument of Republic 436b-441c are good arguments that reasonable Greeks should have accepted (what about us?)

• Protagoras
• KRS: Knowledge Reigns Supreme
  
• "If someone were to know what is good and bad, then he would not be forced by anything to act otherwise than knowledge dictates" (Protagoras 352c4-6)
• ' anything' here specifically includes emotions, pleasure, etc.
• 'force' refers to things coming from inside the agent
  
• Key Assumptions of KRS:
• Knowledge of good and bad would consistently offer an answer to the question "what would be the best thing to do in this circumstance"
• Such knowledge would be fully in control of the agent's actions.
• In other words, no one would/could act contrary to such knowledge.
• What is more, no one would even want to do so.
  
• The knowledge in question is treated quantitatively: a matter of measurement and weighing of alternatives against each other.
• People may still hold confused or mistaken views about appearances, but having such knowledge would make such views powerless.
• No one can have such knowledge and still follow mistaken appearances.
• "Socratic Intellectualism": the agent believes that everything that agent desires to do aims at something that is at least no worse than the other options the agent thinks are available.
• Note that this applies generally: whether or not one has the knowledge of good and evil
• Socrates thinks that this is just what humans do: everyone is Socratically intellectual.
  
• Alternative to KRS and SI: one could believe that some people desire to do things that they believe it would be better not to do, and that some good people have regulated their beliefs and emotions etc. in such a way that what they desire to do is in line with what they think it is good to do.
• But Plato upholds KRS in the Protagoras

• Akrasia, aka "weakness of the will"
• in popular Greek morality, this is the notion that sometimes a person might believe that doing X is not really the best available option but do it anyway, because that person is influenced by pleasure, emotion, etc.
• it is not necessarily the case that at the time the person does X, the person is aware of her belief that it is not the best course of action.
• we would call it a "lapse of judgement"
• or perhaps the person just has no particularly carefully thought out notion that X is not the best option, but does it under the influence of emotion, pleasure etc.

• In the Protagoras, says Lorenz, Plato does not argue that akrasia is impossible.
• What Plato claims is rather that one cannot simultaneously believe that X is not the best available option and still do X.
• Plato thinks that every case that looks like such a case is actually a case of ignorance
• Specifically, that person is ignorant of good and bad.
• Knowledge of good and bad would protect us from such actions, says Socrates, and that is why he wants to discover it.

• Gorgias
• Overall, Plato is an intellectualist in the Gorgias as well.
• Socrates says that one cannot simultaneously know what is just and what is unjust and not do the just or prefer the unjust.
• One cannot even want to do the unjust if one knows what is just.
• Socrates seems to assume that all desires come with the belief that it is better to pursue the desire than not to.
• Thus knowing what is good and bad, just and unjust, motivates one to do it.
• Knowledge steers desires. KRS
  
• Someone who claims to want to do something that is bad for that person is wrong.
• The person may be mistaken about what is good and bad.
• Or a person may be mistaken about what she really wants.
• Socrates is perfectly happy telling people that they do not know what they really want!
• They may claim to want to do X, and Socrates thinks they may be mistaken about wanting to do X.
• That is because their desire to do X rests on the (mistaken) belief that X is the best option (or at least as good as any other option)
  
• One can only do what one wants to do by doing something that is really good for one (or at least the best option).
• Unwise or ignorant people are in the situation of having many mistaken desires: they think they want things that they do not really want.
• But in the Gorgias, the character Callicles suggests that people should enlarge their appetites and indulge them: the overall aim of life is to habitually have the greatest satisfaction of the largest desires possible. Socrates objects to this idea.
• Many desires are part of habits: it is entirely possible to have a habit while knowing that one should break it.
• Such desires can be intense.
• Callicles suggests that those who do not pursue such desires are wimps who are hamstrung by a sense of shame or some other scruple.
• It seems that Socrates has the upper hand, but Callicles' ideas pose a reasonable threat.
• Also, it should be noted that if Socrates is right, it would make no sense to use corporal punishment, because doing wrong is simply a matter of ignorance, and as such, it merits education, not whipping.
• And yet even in the Gorgias, Socrates seems to be willing to allow for corporal punishment. What would it be for if not to break irrational habits? That is to say, habits that don't follow Socratic intellectualism?

• Phaedo
• Soul as harmony and why Socrates rejects it:
• The harmony theory is that the soul is a particular arrangement of bodily parts.
• No arrangement could oppose the parts that compose it (93a8-9, 94c3-7)
• This is the key principle: what does it mean and should we accept it?
• What would it mean for an arrangement to oppose parts that compose it?
• Could it be that, for instance, an arrangements of items in which one part is acting difficult can be "put in order" by the arrangement? Well, it makes no sense to think that the arrangement itself could do so: the arranging force, if there is one, would have to do so, but the arranging force is not the arrangement.
• I can't think of a reason why this principle is wrong.
  
• The soul frequently opposes the bodily parts (94c10-d2) and their affections (94b7-c1).
• presupposes that bodily parts are able to form things like emotions and desires, a fairly common Homeric view: think of Plato's quotation of Homer in the Republic and the Phaedo in which Odysseus chides his heart for its desire to slaughter the serving maids who have dishonored his household.
  
• Therefore, the soul cannot be an arrangement of bodily parts.
• In opposing the harmony theory of the soul, Socrates accepts a sort of psychological conflict which Socratic intellectualism has a harder time accepting
• Namely, in the Odysseus example he uses, Odysseus a) knows it would be a bad idea to kill the maids at that time, but b) nonetheless DESIRES to do so.
  
• Remember that Socratic intellectualism says that: the agent believes that everything that agent desires to do aims at something that is at least no worse than the other options the agent thinks are available.
• Odysseus DESIRES to kill them while KNOWING it would be worse than another available option.
• Can Socrates keep his intellectualism and believe that the Odysseus example is something that really occurs in people?
• Yes, if he believes that one's bodily desires and emotions do not necessarily have any force on the soul: the soul decides what to do.
• This seems a bit unstable (but realistic?): it means that people find themselves bodily desiring to do things that their soul vetoes.
  
• Is the soul infallible in its vigilance?
• Being unstable makes it unattractive because it leaves us vulnerable, but it does not make it illogical or otherwise objectionable as a theory.
• So Scorates can keep his intellectualism in spite of the presence of contrary bodily desires
  
• as long as the idea of "bodily desires" works.
  
• Soul as unitary (incomposite)
• It is naturally appropriate for what has been combined and what is a composite to undergo this, to be divided up in the way in which it has been combined. But if something turns out to be incomposite, for that alone, if for anything, it is appropriate not to undergo these things. Phaedo 78c1-5.
• What the Republic sees as conflicts within the soul, the Phaedo sees as conflicts between the body and the soul.
• In the Phaedo, the body can have emotions and desires
• In the Republic, the body cannot: desires and emotions belong to the soul.
• There is a problem with immortality if the soul turns out to be tripartite, as it does in the Republic:
• A thing that is composite is subject to dissolution back into its parts.
• In Republic 10, Socrates confronts this: it is not clear whether his answer is that the composition of the soul is of such a quality that it does not dissolve or whether his answer is that once separated from the body, the rational part of a philosopher simply has no appetitive and thumetic parts any more, and the rational part is the only immortal part of the soul.

Note: I have come to think that Lorenz fails in his effort to defend the argument for the tripartition of the soul: he promised more than he delivers. He makes some good points, but they do not convince me that Plato has fully justified tripartition. That does not mean that Plato's position is wrong or that he does not have a good argument for it: it means I believe that Lorenz did not succeed in finding it.
Among thoughts his reading has provoked:  
Are all desires attractions? or are aversions and repulsions also part of the appetitive part? Plato never mentions an aversion as being explicitly part of the appetitive part, but Glaucon is worried that the spirited part may be part of the appetitive part, and Socrates thinks it is reasonable to argue against this idea. Socrates does not do so by simply saying "all appetites are attractions, so no aversion can qualify." Thus Socrates may allow that aversions can be part of the appetitive part. (on a side note, my aversion to, say, rotten meat, does not seem to be rational: rather it seems physical, much like thirst, but that's just me perhaps). BUT, if all desires are attractions, then the Leontius example shows very clearly that Spirit is not an appetite. Otherwise, I am not sure what it shows, although it is clear that Glaucon and Socrates think it shows that Spirit is not an appetite.

Plato's concern that thirst is not for good drink, but just for drink has often puzzled me. He makes heavy weather of it, but it seems to do little work as an argument or piece of an argument. Lorenz somehow does not connect the dots, but the Socrates who follows KRS and Socratic Intellectualism would clearly claim that any desire is for something that the agent thinks is good and the agent will only desire it if she thinks it is good. The idea that thirst by itself is not for good drink may be meant to block that move.

What follows is far less close to Lorenz' chapter than the preceeding notes.

• Republic:
• Clearly Plato has abandoned KRS and Socratic Intellectualism: he thinks that people often have irrational desires to do things which their rational part says they should not do.
• In its place, he has a theory that claims three parts of the soul.
• The appetitive part is unruly and selfish and should not be in charge
• putting it in charge will not even satisfy the appetitive part itself, perhaps: it could just lead to chaotic unplanned pursuit of the desire of the moment.
  
• The spirited part too is unruly and selfish and should not be in charge
• The rational part, however, is capable of considering the other two parts' well-being
• But later in the Republic, it will be revealed that if the rational part is in charge, the person's appetitive part will wither as will the spirited part, and so the ascendancy of the rational part seems to be a tyranny too...unless there is some way to establish that the rational part has good goals while the appetitive and spirited part has bad goals...
• Why should we believe that the rational part has good goals?
• It's primary desire is for truth.
• What's good about truth?
  
• As for my opinion, if you care to hear it, I buy the point that the rational part should be in charge, because it is capable of considering the other two parts' well-being as well as the well-being of the whole person. Insofar as I think that I consist of appetitive, spirited and rational part, it seems obvious to me that the rational part should be in control, but I don't buy the idea that my goal in life should be the minimization of my bodily appetites or that my body is the source of evil and that I should avoid all things bodily for eternal salvation. As far as I'm concerned, I die and death ends it, so there is no particular other-life-oriented point to rationality. Putting your executive function fully in charge is just the most efficient way to organize one's life for optimum satisfaction. Sort of by definition: the other parts have no capacity to figure priorities out. That is simply what the executive part does: for judgements, set up hierarchies, etc.