In the Republic, Plato introduces a new understanding of the human soul. The soul now has three parts: "reason" (λογιστικὸν) "spirit" (θυμοειδές), and "appetite (ἐπιθυμητικόν)." All parts of the soul have desires, but desire in appetitive and spirited parts is not a matter of belief about what is good and what is bad. The desires in these parts arise independently of any beliefs about the good.

In the Tripartite Theory of the Soul, Plato abandons the Socratic intellectualist theory Socrates advocates in the Protagoras.

"There is reason to believe that Socrates thought that there is no such thing as acting against one's own better judgment. What does happen is that reason in certain circumstances gets confused and, instead of holding on to its better judgment, follows some other judgment. If reason knew the truth, it could never get confused in this way. Thus, according to Socrates, such cases reveal nothing but a failure of reason which in its weakness does not hold on to the true belief, but accepts a false one and acts on it. Plato, Aristotle, and their followers, on the other hand, believed that such cases could not be explained as purely intellectual failures, that one had to assume that besides reason there is an irrational part of the soul with its own needs and demands which may conflict with the demands of reason and which may move us to act against the dictates of reason, if reason has not managed to bring the irrational part of the soul firmly under its control" (Michael Frede, "The Stoic Doctrine of the Affections of the Soul," 96).

The Argument from Opposites

Sometimes a person is thirsty but refuses to drink. How is this possible? According to the Tripartite Theory, there are two desires in play. One stems from appetite. It arises naturally in reaction to events in the body. In the absence of a desire from reason, this appetitive desire would motivate the person to drink. Reason, however, in the example, has the belief that in the circumstance drinking is not what is best. If reason rules, the connection between the appetitive desire and action is interrupted. Reason overrides appetite.

"Is there something in the soul of those who are thirsty but refuse to drink, something bidding them to drink and something different, something forbidding them, that overrides the thing that bids them to drink? And doesn't the thing that forbids in such cases come into play, if it comes into play, as a result of calculation, while what drives and drags them to drink is a result of feelings and diseases? Hence isn't it right for us to claim that they are two, and different from one another? We'll call the part of the soul with which it reasons the λογιστικὸν and that with which it lusts, hungers, thirsts, and feels other appetites, the irrational (ἀλόγιστόν) and appetitive part (ἐπιθυμητικόν), companion of indulgences and pleasures" (Republic IV.439c-d).

Socrates argues for this understanding in terms of a principle about opposite motions.

"It is obvious that the same thing will never do or suffer opposites in the same respect in relation to the same thing and at the same time. So that if ever we find this happening we shall know that it was not the same thing but a plurality" (Republic IV.436b-436c).

The suggestion is that desire and aversion are opposite motions of the soul. Desire is a motion toward, and aversion is a motion away. If a person is thirsty, he has a motivation to drink. If he thinks that drinking is not in his best interest, he also has a motivation not to drink. If this desire and aversion are opposite motions, then given the principle about opposite motions, it follows that this desire and aversion are motions of different things in the person. Since human beings are psychological beings, and thus do whatever they do because of states and processes in their psychology, Socrates concludes that the human psychology has at least two parts. The appetitive part of the soul has the desire, and the part of the soul that reasons has the aversion.

The argument from opposites is questionable. One may wonder whether desire and aversion are motions and whether the soul is something whose parts can move at all.

Aristotle raises this objection against Plato. "It is perhaps better to say, not that the soul pities or learns or thinks, but that person does in virtue of the soul" (Aristotle, On the soul 408b13-15). To understand Aristotle's point, what it is for the soul to be a starting-point of movement will need to be carefully considered. We will do that later in the course.

The third part of the soul

In addition to the part with reason and the part with appetites, Socrates argues for a third part of the soul: the "spirited" part (θυμοειδές). "Of the spirit (θυμοῦ), that with which we feel anger, is it a third, or would it be the same as these [we have distinguished]" (Republic IV.439e)? Socrates says that some part of the soul conflicts with appetite in the case of Leontius (Republic IV.439e-440a), that children have spirit, but that "as for reason, some of them, to my thinking, never participate in it, and the majority quite late" (Republic IV.441a-b). The argument seems to be that examples of the conflict in the case of Leontius are seen in children and in animals (Republic IV.441b), that animals lack reason all together, and that reason does not play a controlling role in the actions of children.

Reason Rules in a Harmonious Organization of the Parts of the Soul

Given the Tripartite Theory of the Soul, there are different possible organizations among the parts of the soul. The proper organization is the one in which reason rules, spirit is reason's ally, and appetite is suppressed. When the parts are so organized, they are in "harmony." Since reason knows the good, a human being whose soul is in "harmony" acts for the sake of the good.

"It is appropriate for the reasoning part (λογιστικῷ) to rule (ἄρχειν), since it is wise and exercises foresight on behalf of the whole soul, and for the spirited part to obey and be its ally" (Republic IV.441e4-6).

How the Parts of the Soul without Reason generate Action

It is part of the point of the Tripartite Theory that a human being can act without reason. This means it is possible for a human being to act in terms of the appetitive part and the spirited parts of the soul. For this to be possible, it must be possible for these parts of the soul to represent the states of affairs they are motivated to bring about.

What are these representations? Are they beliefs about how the world is?

One might think that that these representations are beliefs and that belief is a cognitive state that can belong to all the parts of the soul. In the parts of the soul without reason, one might think that sensation and imagination form the beliefs. These parts of the soul would accept these representations uncritically. It would be impossible for them to reject a representation as mistaken because they cannot reason about whether these representations are true. They would have to accept whatever representations sensation, imagination, and memory present.

Is this how Plato understands how appetite and spirit can cause action?

In Book X of the Republic, in the context of the argument against imitation and imitative poetry, Socrates seems to commit himself to the view that the parts of the soul without reason can have beliefs. He seems to argue that sometimes there is a belief in the part of the soul with reason that is opposite to a belief in one of the parts of the soul without reason.

"The same magnitude viewed from near and from far does not appear equal. And the same things appear bent and straight to those who view them in water and out, or concave and convex, owing to similar errors of vision about colors. There is every confusion of this sort in our souls, but measuring and numbering and weighing prevent the domination in our soul of the apparently greater or less or more or heavier, and give the control to that which has reckoned and numbered or even weighed. This would be function of the part of the soul that reasons and calculates (λογιστικοῦ). Sometimes, when this has measured and declares that certain things are larger or that some are smaller than the others or equal, there is at the same time an appearance of the contrary. But we said that it is impossible for the same thing at one time to hold contradictory opinions about the same thing. It follows, then, that the part of the soul that opines (δοξάζον) in contradiction of measurement could not be the same with that which conforms to it. Further, that which puts its trust in measurement and reckoning must be the best part of the soul. That which opposes it must belong to the inferior elements of the soul. This, then, was what I wished to have agreed upon when I said that poetry, and in general the mimetic art, produces a product that is far removed from truth in the accomplishment of its task, and associates with the part in us that is remote from intelligence, and is its companion and friend for no sound and true purpose" (Republic X.602c-603b).

Beliefs are an Achievement of Reason

In the Timaeus, which is traditionally thought to be a late dialogue, Plato seems to have a different view of these representations. Timaeus says that the appetitive part of the soul is "completely devoid of belief (δόξης), reasoning (λογισμοῦ), and thought (νοῦ)" (Timaeus 77b3-6). The suggestion in the Timaeus is that belief is strictly an achievement of reason.

One way to understand the different points of view about "belief" is to suppose that Plato came to thnk that belief is an achievement of reason and that what looks like a belief in the parts of the soul without reason is something else, such as a perception of the senses. If this interpretation is correct, then one would expect Plato to work out this new understanding in some dialogue.

In fact, Plato does work out this view. It happens in the Theaetetus, which is traditionally thought to be a late dialogue. (Theaetetus was an Athenian mathematician who work in the theory of incommensurable quantities. The suggestion in the dialogue is that Theaetetus died died from wounds and dysentery on his way home after fighting in an Athenian battle at Corinth.)

The Theaetetus is about knowledge, what it is (145e). Theaetetus suggests that "knowledge is nothing else than perception" (οὐκ ἄλλο τί ἐστιν ἐπιστήμη ἢ αἴσθησις) (151e). In 184b-187a, to refute this understanding of knowledge, Socrates says that the soul grasps some things through the senses and some things in some other way. We perceive color, for example, through the eyes and sound through the ears. Further, Socrates says that a thought about color and sound together would not be something that the soul perceived through a sense. A thought whose content is that a color and a sound both exist, for example, is not something the soul acquires through a sense. Instead, Socrates says that soul achieves this thought by itself. The soul directly grasps being, likeness, differences, and so on. This means that perception grasps and is aware of perceptual features of the world but does not predicate anything of these features. The application of predicates is an achievement of the capacity to form beliefs. At 186b, Socrates says that the soul perceives hardness through the sense of touch; however, to predicate hardness of the thing that is hard and thus to form the belief that the thing is hard, the soul must grasp that hardness is, that softness is, and that they are opposites. In this way, belief is an accomplishment of reason. "When the soul thinks, it is conversing with itself, asking itself questions and answering, affirming and denying. When it has arrived at something definite, whether slowly or suddenly, and is at last agreed and is not in doubt, we call that its belief (δόξαν); and so I define forming a belief as talking and a belief as talk which has been held, not with someone else, nor yet aloud, but in silence with oneself" (Theaetetus 189e-190a).

Not all Representations in the Soul are Beliefs

This understanding of belief raises a question for the Tripartite Theory of the Soul. The parts of the soul without reason can generate action. So they must have representations of the world that can move a human being to act in a specific way. If these representations are not beliefs, what are they? The answer seems to be the they are a matter of perception, imagination, and memory.

The Philebus (which is also traditionally thought to be a late dialogue) provides some explanation of a way in which the parts of the soul without reason are capable of providing representations of the world that are motivating. The topic in the Philebus is the good. Philebus holds that the good is the same for both humans and animals: namely, to enjoy oneself, to be pleased, to be delighted. To determine whether he is correct, it is necessary to investigate pleasure. Socrates says that pleasure that belongs to the soul alone depends on memory. (33c.) To explain why this is true, Socrates provides accounts of perception, memory, and desire. Perception, he says, involves the soul and the body. (34a.) Memory, in turn, is the preservation of perception. (34a.) Socrates says that hunger and thirst are desires. (34d-e.) Such desires occur in the presence of depletion and are for the opposite, replenishment. (35a.) For a living thing to have the desire for a given replenishment, the living thing must represent the replenishment in some way. (35b.) According to Socrates, memory supplies the representation. Memory supplies the representation that represents the object of the desire. (35b-d.) Socrates does not make the point explicitly, but it seems possible to think that he supposes that memory also provides the representation of how to acquire the object of the desire.

In this discussion of pleasure in the Philebus, Socrates makes it clear that this understanding applies to both animals and human beings. Only human beings have beliefs because only human beings have reason. Socrates suggests that beliefs are accompanied with the images that alone are the means of representation in animals. He compares the soul to an illustrated book. Forming a belief is like writing a sentence in the soul. (38e-39a.) In addition to the "writer" in the soul, Socrates says that there is an illustrator who makes illustrations of the words the writer has written. (39b.)

How Reason Rules Spirit and Appetite

It seems that reason somehow uses the illustrations (discussed in the Philebus) to control the parts of the soul without reason.

In the Timaeus, the gods who fashioned the mortal parts of the soul knew that appetitive part would not understand reasons but would be enticed by "images and appearances" (εἰδώλων καὶ φαντασμάτων). (71a.) Consequently, they fashioned the appetitive part of the soul in the body in such a way that thoughts could carried down to it from the intellect. This, when appropriate, produces pain and nausea or, alternatively, cheerfulness and serenity. (71b-d.) In the Philebus, in the discussion of pleasures of anticipation, there is an indication of what happens. In pleasures of anticipation, when memory supplies the image of replenishment (that provides the object of the desire), the person takes anticipatory pleasure in the expected replenishment. (36b.) This suggests a way for reason to rule. Reason uses the mechanism that underlies anticipatory pleasure and pain to control the parts of the soul without reason.

Consider a compulsive behavior, such as smoking. The appetitive part of the soul may form a liking for smoking because it is pleasurable. Over time, this liking may become strong. If, at some point, reason discovers that smoking is bad, this belief alone may not be enough to prevent the liking in the appetitive part of the soul from issuing in the behavior of smoking. Reason, however, can do something more. It can form beliefs about and imagine the painful consequences of smoking so that the appetitive part of the soul associates the pain depicted in these images with smoking and thus takes less anticipatory pleasure in smoking. Reason, in this way, can use imagination to recalibrate how much the appetite likes smoking.

Furthermore, aside from the factual questions of whether human beings have a soul and whether this soul has parts with and without reason, it is clear that a psychological phenomenon something like the one Plato envisions is real. A person may engage in activity that has certain risks. He may enjoy the activity at the time. Subsequently, he may think about what might have happened and how bad that would have been. This thinking has the power to change his initial liking for the activity. Alternatively, instead of thinking about what might have happened, he might think about the probable long-term consequences of the activity and about how bad they are. This thinking too has the power to change his initial liking for the activity. I assume that this idea is behind the TV campaigns to stop such activities involving tobacco and certain drugs by showing the viewer images that portray, in a horrific manner, the long-term negative consequences linked to smoking and drug use.

Perseus Digital Library:
Plato's Republic, Theaetetus, Timaeus, Philebus
Henry George Liddell, Robert Scott, A Greek-English Lexicon: