Aporia is the Greek term for the state of helplessness—the inability to proceed—that ends all of Plato’s early dialogues. Through his pointed questioning, Socrates succeeds in showing that his interlocutors have no appropriate definition for the topic under consideration (be that topic piety, love, courage, justice, or whatever else), but nor is he able to supply one himself. In Book 1 of The Republic Socrates brings his friends to a state of aporia on the topic of justice, but then in the next nine books he manages to move beyond the aporia and give an actual answer to the question at hand.


Appetite is the largest aspect of our tripartite soul. It is the seat of all our various desires for food, drink, sexual gratification, and other such pleasures. It contains both necessary desires, which should be indulged (such as the desire to eat enough to stay alive), unnecessary desires, which should be limited (such as the desire to eat a ten-pound sirloin steak at every meal), and unlawful desires, which should be suppressed at all costs (such as the desire to eat one’s children). Though the appetite lusts after many things, Plato dubs it “money-loving,” since money is required for satisfying most of these desires. In a just man, the appetite is strictly controlled by reason and reason’s henchman, spirit.


Plato divides his just society into three classes: the producers, the auxiliaries, and the guardians. The auxiliaries are the warriors, responsible for defending the city from invaders, and for keeping the peace at home. They must enforce the convictions of the guardians, and ensure that the producers obey.


Belief is the second-lowest grade of cognitive activity. The object of belief is the visible realm rather than the intelligible realm. A man in a state of belief does not have any access to the Forms, but instead takes sensible particulars as the most real things.


Elenchus is the Greek term for Socrates’s method of questioning his interlocutors. In an elenchus, he attempts to show that their own beliefs are contradictory, and thus to prove that they do not have knowledge about some topic about which they thought they had knowledge.


When something is an empirical question, that means that the question can only be settled by going out into the world and investigating. The question, “What percentage of the population of the United States likes ice cream” is an example of an empirical question, which can only be answered through empirical investigation. The question “What is the square root of two,” on the other hand, is not an empirical question. In order to answer this question all you have to do is think about the mathematics involved; you do not have investigate evidence in the world.


The branch of philosophy concerned with knowledge, belief, and thought. Epistemological questions include: What is knowledge? How do we form beliefs based on evidence? Can we know anything?


According to Plato’s metaphysical theory, there is an aspect of reality beyond the one which we can see, an aspect of reality even more real than the one we see. This aspect of reality, the intelligible realm, is comprised of unchanging, eternal, absolute entities, which are called “Forms.” These absolute entities—such as Goodness, Beauty, Redness, Sourness, and so on—are the cause of all the objects we experience around us in the visible realm. An apple is red and sweet, for instance, because it participates in the Form of Redness and the Form of Sweetness. A woman is beautiful because she participates in the Form of Beauty. Only the Forms can be objects of knowledge (that is, Forms are the only things we can know about).

Form of the Good

Among the Forms, one stands out as most important. This is the Form of the Good. Plato is unable to tell us exactly what the Form of The Good is, but he does tell us that it is the source of intelligibility and of our capacity to know, and also that it is responsible for bringing all of the other Forms into existence. He compares its role in the intelligible realm to the role of the sun in the visible realm. The Form of the Good is the ultimate object of knowledge; it is only once one grasps the Form of the Good that one reaches the highest grade of cognitive activity, understanding. Therefore, it is only after he grasps the Form of the Good that a philosopher-in-training becomes a philosopher-king.


Plato divides his just society into three classes: the producers, the auxiliaries, and the guardians. The guardians are responsible for ruling the city. They are chosen from among the ranks of the auxiliaries, and are also known as philosopher-kings.


Hesiod was a famous Greek poet. His long poem Works and Days outlines the traditional Greek conception of virtue and justice.


Imagination is the lowest grade of cognitive activity. Someone in the state of imagination takes mere images and shadows as the most real things. Probably, this means that such a person derives his ideas about himself and the world from products of art, such as poetry in Plato’s day and movies and television in our own. See also Belief, Thought, Understanding.

Instrumental Reason

Instrumental reason is reason used to attain some end, by engaging in means-end analyses. These ends are dictated by a part of the soul such as appetite or spirit, or even reason itself.

Intelligible Realm

Plato divides all of existence up into two parts: the visible realm and the intelligible realm. The intelligible realm cannot be sensed, but only grasped with the intellect. It consists of the Forms. Only the intelligible realm can be the object of knowledge.


Kallipolis is the Greek term for Plato’s just city.


According to Plato, knowledge can only pertain to eternal, unchanging truths. I can know, for instance that two plus two equals four, because this will also be the case. I cannot know, however, that Meno is beautiful. For this reason, only the intelligible realm, the realm of the Forms can be the object of knowledge. See also Opinion.

Lover of Sights and Sounds

Lovers of sights and sounds is Socrates’s term for the pseudo-intellectuals who claim to have expertise regarding all that is beautiful, but who fail to recognize that there is such a thing as the Form of the Beautiful, which causes all beauty in the visible realm. Socrates is adamant that lovers of sights and sounds be distinguished from philosophers, who grasp the Forms, and thus have knowledge. Lovers of sights and sounds have no knowledge, only opinion.


The branch of philosophy concerned with asking what there is in the world. The theory of Forms is a metaphysical theory, as is the theory of the tripartite soul.


Since only eternal, unchanging truths can be the objects of knowledge, all other truths are relegated to opinion. Opinion is the highest form of certainty that we can hope for when it comes to the visible realm, the realm of sensible particulars.


The philosopher-king is the ruler of the kallipolis. Also called guardians, philosopher-kings are the only people who can grasp the Forms, and thus the only people who can claim actual knowledge. Since the philosopher-king yearns after truth above all else, he is also the most just man.


A Greek term meaning “the desire to have more,” pleonexia refers to the yearning after money and power. In Book I, Thrasymachus presents the popular view that justice is nothing more than an unnatural restraint on our natural pleonexia.


Plato divides his just society into three classes: the producers, the auxiliaries, and the guardians. The producing class is the largest class of society; it is a catch-all group that includes all professions other than warrior and ruler. Framers and craftsmen are producers, as are merchants, doctors, artists, actors, lawyers, judges, and so forth. In a just society, the producers have no share in ruling, but merely obey what the rulers decree. They focus exclusively on producing whatever it is that they are best suited to produce (whether that be metal work, agriculture, shoes, or furniture).


Reason is one aspect of our tripartite soul. It lusts after truth and is the source of all of our philosophic desires. In the just man, the entire soul is ruled by reason, and strives to fulfill reason’s desires. See also Appetite, Spirit.

Sensible Particular

Sensible particulars are the objects that we experience all around us—trees, flowers, chairs—any physical objects. They are “sensible” because we can sense them with our sight, smell, hearing, taste, and touch; they are “particular,” because they are particular items that undergo change over time, rather than universal, unchanging ideas. According to Plato’s metaphysical picture, the visible realm is made up of sensible particulars. According to his epistemological picture, sensible particulars cannot be objects of knowledge but only of opinion.

The Sophists

The Sophists were teachers-for-hire who educated the wealthy men of Athens in the fifth century BCE. Though they were a diverse group with diverse opinions, they tended to share a disregard for the notion of objective truth and knowledge. This disregard extended to the notion of objective moral truth, which means that they did not believe in such a things as “right” and “wrong.” One of the guiding motivations in all of Plato’s work was to prove the Sophists wrong: to show that there is such a thing as objective truth, and that we can have knowledge of this objective truth.


The principle of specialization states that every man must fulfill the societal role to which nature best suits him, and should refrain from engaging in any other business. Those naturally suited to farm should farm, those naturally suited to heal should be doctors, those naturally suited to fight should be warriors, those naturally suited to be philosophers should rule, and so on. Plato believes that this simple rule is the guiding principle of society, and the source of political justice.


Spirit is one aspect of our tripartite soul. It is the source of our honor-loving and victory-loving desires. Spirit is responsible for our feelings of anger and indignation. In a just soul, spirit acts as henchman to reason, ensuring that appetite adheres to reason’s commands.


Thought is the second-highest grade of cognitive activity. As with understanding, the objects of thought are the Forms of the intelligible realm. Unlike understanding, though, thought can only proceed with the crutches of images and hypotheses (i.e. unproven assumptions). See also Belief, Imagination, Understanding.

Tripartite Soul

According to Plato, the human soul has three parts corresponding to the three classes of society in a just city. Individual justice consists in maintaining these three parts in the correct power relationships, with reason ruling, spirit aiding reason, and appetite obeying.


Understanding is the highest grade of cognitive activity. Understanding involves the use of pure, abstract reason, and does not rely on the crutches of images and unproven assumptions. Understanding is only achieved once the Form of the Good is grasped. See also Belief, Imagination, and Thought.

Visible Realm

Plato divides existence up into two realms, the visible realm and the intelligible realm. The visible realm can be grasped with our senses. It is comprised of the world see around us—the world of sensible particulars. The objects which comprise the visible realm are not as real as those which comprise the intelligible realm; in addition, they are not the proper objects of knowledge (i.e., we cannot “know” anything about them), but of opinion.