Socrates concludes Book IV by asserting that justice amounts to the health of the soul: a just soul is a soul with its parts arranged appropriately, and is thus a healthy soul. An unjust soul, by contrast, is an unhealthy soul. Given this fact, we are now in a position to at least suspect that it pays to be just. After all, we already admitted that health is something desirable in itself, so if justice is the health of the soul then it too should be desirable. Plato feels that he is not ready just yet to make the argument in favor of justice’s worth. He puts off the definitive proofs until Book IX.

Analysis: Book IV, 435d-end

The word justice is applied by Plato to both societies and individuals, and Plato’s overall strategy in The Republic is to first explicate the primary notion of political justice, then to derive an analogous concept of individual justice. Plato defines political justice as being inherently structural. A society consists of three main classes of people—the producers, the auxiliaries, and the guardians. The just society consists in the right and fixed relationships between these three classes. Each of these groups must do the appropriate job, and only that job, and each must be in the right position of power and influence in relation to the other.

In this section, Plato sets out to show that the three classes of society have analogs in the soul of every individual. In other words, the soul, like the city, is a tripartite entity. The just individual can be defined in analogy with the just society; the three parts of his soul are fixed in the requisite relationships of power and influence. In order to make this claim work, Plato must prove that there really are three parts of the soul.

There are two distinct legs of the argument for the tripartite soul, and the relationship between them is obscure. The first leg attempts to establish the presence of three distinct sets of desire in every individual. The second leg argues that these three sets of desire correspond to three distinct sources of desire, three distinct parts of the soul. The ultimate conclusion is that every individual has a tripartite soul. Plato has to classify the desires, because setting out to prove that there are three distinct parts of the soul without first establishing that there are these three types of desire, would not be as stylistically effective or compelling. The first leg bridges the transition from the societal to the individual level by showing that group properties stem from individual properties.

Why is it important for Plato to demonstrate that the three types of desire present in every individual correspond to three independent sources of desire? Why would it not be sufficient to maintain that these three forces are manifested at different times by the same subject, but do not correspond to three distinct parts of the soul?

This distinction allows the three types of desire to be exerted simultaneously. Political justice is a structural property, consisting in the relationships of three necessary parts. The relationships constituting political harmony are fixed and static in the same sense as the mathematical ratios that constitute musical harmony. In the individual, though desires come and go, the relationship between the different sets of desires remains fixed. The three-part division of the soul is crucial to Plato’s overall project of offering the same sort of explication of justice whether applied to societies or individuals.

Plato begins his argument for the tripartite soul by setting up a criterion for individuation. The same thing cannot be affected in two opposite ways at the same time (436c). As pairs of opposites, he includes “assent and dissent, wanting to have something and rejecting it, taking something and pushing it away” (437b). Plato argues for the truth of this claim by bringing analogies from the behavior of bodies—a method which may seem illegitimate, given that he wants to use the principle to apply to aspects of the soul (in particular, opposing desires), not to physical objects.

In order to make the leap from observations about forces and desires to conclusions about parts of the soul, Plato relies throughout the argument on a suppressed metaphysical claim. Where there is desire, there is the agent of desire: the thing which desires. Using this premise and the criterion for individuation, he will arrive at three distinct parts of the soul, corresponding to the three aspects he has identified within the city.

Plato first tries to establish the existence of a purely appetitive part of the soul using this method. Thirst is a desire. There is a subject of this desire. Thirst is a desire for unqualified drink—that is, no particular kind of drink, just drink (437e). Now comes a logical digression, the aim of which is to preclude the combination of appetitive and rational forces in the same subject. The outcome of the logical digression is that if the truth about a is relative to the truth about b, then if b is qualified in a certain way, a must be analogously qualified (438a-e). Therefore, the agent of thirst desires drink unqualified (439b).

Because the agent desires unqualified drink rather than good drink, healthful drink, etc., it cannot be argued that this subject is a combination of appetitive and rational forces. The subject corresponding to thirst is characterized by pure animal urge, with no rational discrimination. If, on the other hand, the desire for drink were theoretically inextricable from the desire for good or healthy drink, there would be no pure appetite, and correspondingly no purely appetitive subject.

The desire for drink is representative of a whole class of desires which stem from the same agent. Other appetitive desires include hunger and lust for sex. The subject which desires unqualified food, drink, and sex is the appetitive part (437c). Plato feels no need to establish that the same agent is responsible for these various, though obviously related, desires. No reason is demanded for the identification of agents of desire, only for their separation.

Plato next attempts to isolate the rational part of the soul. He says that if there is a desire which opposes the appetitive desire, there is another, separate agent of desire. He then makes the empirical claim that there are sometimes thirsty people who do not wish to drink (439c). Therefore, there is an agent which desires to drink, and another agent which desires not to drink.

Plato then makes another empirical claim—that desires opposing the appetites always come from rational thought (439d). He concludes that the second agent’s desires come from rational thought. He now believes himself to have identified a purely appetitive and a purely rational subject.

Plato is not justified in asserting that reason always opposes appetite. It is fairly easy to conceive of a situation in which spirit, rather than reason, would oppose appetite. Plato does not need to make as strong a claim that only reason opposes appetite. Instead, he could give an example of an anti-appetitive desire which does, in fact, happen to come from reason—for instance, not wanting the drink because it is unhealthy. He could then conclude that there is an agent other than appetite and that this agent’s desires come from rational thought. Adding the extra claim that all desires which oppose appetitive desires stem from reason, is unnecessary, false, and inconsistent with a later step in this argument which shows spirit opposing the appetite.

It would be more problematic if one could imagine a situation in which two appetites are opposed to one another. Plato would respond, however, that it is reason which tells us that two conflicting appetitive desires are mutually exclusive, forcing us to view them as opposing desires.

Having argued for the existence of two different parts of the soul—one appetitive and the other rational—Plato needs only to establish that there is a third, spirited part of the soul in order to complete the analogy with the city. Once again, he begins this project by establishing the existence of a third branch of desire, as well as an agent of that desire. Anger and indignation are desires. There is an agent of these desires. Next, he tries to prove that this third agent does not reduce to either of the two already established.

He first shows that spirit is not appetite. A man can feel angry at his appetites (440a). The third agent is not the same as the appetitive part. In contrast with the other potential identifications—i.e. reason with appetite, spirit with appetite—the only possible identification Plato contemplates between spirit and reason places spirit in the position of reason’s henchman, carrying out the desires reason dictates. Plato, therefore, does not use the regular criterion of individuation to distinguish spirit from reason. Instead, he attempts to show that spirit cannot amount to the henchman of reason because it sometimes acts in reason’s absence. Children and animals have the desires of the third agent without having the reasoning part of the soul (441b). Therefore, the third agent is not the rational part of the soul. Plato concludes that there are three separate parts of the soul: appetite, spirit, and reason.

In what way are these three distinct parts, and in what way do they make up a unified whole? Plato’s argument for a tripartite soul in Book IV, as well as his description of the three parts of the soul in Book IX, depend primarily on identification of the soul and its parts through the desires exerted. Desires are active principles, forces that motivate the passive body. The soul, then, at least here, can be seen as a metaphysical entity which serves as the seat of human activity. The soul is the collection of active principles in a human being.

According to Plato, there are three main “psychological” forces at work in an individual—the force which has as its object physical entities and money; the force which has as its object nonmaterial but worldly entities such as honor and victory; and the force which has as its object the insensible realm of the Forms. These three forces are expressed in desires which correspond to appetite, spirit, and reason. All three of these forces make up one entity—the soul—in that they comprise the collective group of active principles in an individual. Yet they are distinct active principles which operate in different ways and have very different objects.

Because the soul is the seat of human forces, it is clear why Plato thought it appropriate to individuate its parts by demonstrating opposing desires within it. The best way to prove that there are independently working active forces within the soul is to demonstrate these forces exerting themselves in opposition to one another. Clearly the same active force cannot be responsible for the exertion of two opposing forces. Revealing opposing desires amounts to revealing discrete active forces within the collective seat of activity.

Plato uses this criterion of individuation to demonstrate that there are three active forces within the soul. While he does succeed in isolating three types of desire, he does nothing to prove that there are no more than three active forces. Perhaps rather than a tripartite soul, there is really a quadpartite or quinpartite soul. What evidence does Plato have to restrict it to three?

Plato’s tripartite analysis of the soul puts forth at least three quite substantive claims. First, there are psychological agents of desire that possess the forces that act upon the body. Second, the multitudes of desires that an individual possesses can be reduced to three main categories, corresponding to three such psychological agents of desire that control human behavior. Third, the fundamental description of human psychology—that of the “structure of the soul”—has ethical implications and is necessary to an understanding of justice.

While the first and third claims have little currency among modern thinkers, the tripartite division of the individual psyche or soul has remained a viable hypothesis in accounting for internal psychological conflicts in the modern era. It survives, in modified forms, in such modern reincarnations as Freud’s tripartite division between the id, the ego, and the superego.