In Book I of Plato's Republic, Thrasymachus, Glaucon, and Adeimantus provide intellectual foils for Socrates's ethical philosophy. Socrates responds to Thrasymachus's stance, which is essentially that, "the life of an unjust person is better than that of a just one," (p 88; 347e). Thrasymachus goes so far as to state that justice is "noble naivete," and therefore not worth pursuing at all (348c). Glaucon immediately takes the side of Thrasymachus, by stating that the life of an unjust person is "more profitable," at the very least, than the just life (p. 88, 347e). Agreeing with Glaucon's doctrine of self-interest, Adeimantus offers his two bits in Book II of The Republic. Glaucon's brand of justice is that good reputation is the most important thing. A person does not want to be just if those just acts go unnoticed or unrecognized. More importantly, Adeimantus states that a person would be better off committing unjust acts and getting away with them because their reputation would remain intact. Thrasymachus, Glaucon, and Adeimantus each offer sensible, practical, and material counter claims to Plato's (Socrates's) central thesis that Justice is an end in itself and a goal that also brings about Goodness and Happiness. Plato meets the intellectual challenge of Thrasymachus, Glaucon, and Adeimantus in two ways: referring to the concept of the Forms; and referring to the concept of the tripartite division of the soul. The tripartite division of the soul has its macrocosmic counterpart in the polis, and therefore an orderly soul is matched by an orderly society. When Aristotle grapples with the validity and value of both Virtue and Justice, he takes a different approach than Plato does. Aristotle cuts to the heart of the argument by insisting on a distinction between acts that are just and acts that are virtuous. Moreover, Aristotle's ordering of the soul is less rigid than Plato's. Aristotle's argument is more effective for responding to Thrasymachus, Glaucon, and Adeimantus not because he proves his case better, but because he appeals to the logical and ethical framework that Thrasymachus, Glaucon, and Adeimantus work with; whereas Plato does not do that.
Plato is, in modern terms, not on the same wavelength as Thrasymachus, Glaucon, and Adeimantus. Thrasymachus, Glaucon, and Adeimantus are coming basically from a utilitarian perspective in which there are no moral absolutes. With no absolute ethical truth or values, the ends can easily justify the means. It makes sense, then, that Thrasymachus, Glaucon, and Adeimantus do not find inherent value or worth in either justice or virtue. For Plato, Justice and Virtue are Forms. They are inherently and immutable absolute states that must be achieved and attained in order for a human being to experience happiness. Happiness is an end-product, symptom, or a by-product of Justice.
Plato's vision of an orderly universe with the Forms as the absolute and Right expression of all things runs contrary to the metaphysical vision that Thrasymachus, Glaucon, and Adeimantus posit, even though their metaphysic is more immature than that of Socrates/Plato. With regards to the argument over the validity of Justice, Thrasymachus, Glaucon, and Adeimantus simply point to practical matters that Plato finds too mundane to entertain. For example, Thrasymachus points out the value of the self-serving existence; he argues that a self-serving type act can be unjust but that type of injustice is ethically permissible because it preempts injustice done to the self. Thrasymachus is actually completely unconcerned with justice, as he presents ethics as "what's in it for me?" All Plato can say in response is that the abstract notion of Justice is Good and it has inherent value.
Plato's abstract thinking is less effective than Aristotle's long-winded logical debate because the latter provides room for Thrasymachus's mindset. Socrates's commitment to Justice as an abstract Goal has no bearing on Thrasymachus's worldview. Thrasymachus's worldview remains stalwartly materialistic and selfish. If he were in the room, Aristotle would have observed that Thrasymachus is simply enacting the Doctrine of the Mean in his method of calculated thinking. If Justice and Virtue have no clear ends that serve the self, then they have no clear value for the likes of Thrasymachus. Thrasymachus is a political realist. He sees that justice is not an abstract ideal; or even if it were, its ideal would be untenable for human beings. Thrasymachus finds that those in positions of power who wield the sword of justice are the only ones empowered to determine what acts are deemed just, and what acts are deemed unjust. For an individual to act in accordance with some abstract form of Justice, in the spirit of doing Good or being Virtuous, that person is categorically naive, according to Thrasymachus.
Glaucon agrees with Thrasymachus but for different reasons. He is slightly less pessimistic than Thrasymachus and does not categorically dismiss the Form or Reality of Justice or Virtue. Instead, though, Glaucon uses the story of Gyges to show that just acts must have some kind of reward for the person and that reward is honor, glory, and good reputation. Adeimantus chimes in to agree that most people need and want honor and glory when they act Justly. However, he also adds that alternatively, people want honor and glory even when acting unjustly. All three men, Adeimantus, Thrasymachus, and Glaucon, believe that justice is valuable only if it brings personal reward. They also measure that reward with terms Plato would consider mundane. Because Plato cannot level with Adeimantus, Thrasymachus, and Glaucon on their materialistic outlook, his argument is lost on them.
More relevant to what Thrasymachus, Glaucon, and Adeimantus would want to hear, Aristotle suggests that Justice is not an end in itself but a means to acquiring Happiness. Aristotle's principle of the Golden Mean does not contradict Plato's point-of-view, but it does present a more nuanced outlook that would appeal to the likes of Thrasymachus, Glaucon, and Adeimantus. In particular, the doctrine of the Golden Mean permits ethical balance and nuance. Justice is not as absolute as Plato makes it out to be. In Aristotle's ethical worldview and careful analysis, the philosopher notes that some people are simply incontinent (or intemperate), and some are prudent. Each person has a moral character that can become virtuous and good when a fine balance has been achieved within the soul. In his Nichomachean Ethics, Book Four, Chapter 5, Aristotle outlines the five states of the soul. The five states of the soul for Aristotle include craft, scientific knowledge, prudence, wisdom, and understanding. Moreover, Aristotle outlines the three capacities of the soul. Those soul capacities are sense perception, understanding, and desire.
Aristotle is not nearly as utilitarian in his ethical probing as Thrasymachus, Glaucon, and Adeimantus. For Aristotle, Virtue does indeed exist and has an inherent value. Compared with Plato, though, Aristotle does not have as rigid a framework for ethical practice. This is true on the individual level and the collective level. Plato's analysis of the soul is less nuanced than that of Aristotle because Plato only posits a tripartite division of the soul into a hierarchical triangle with Reason at the top, and thumos (vitality, energy, spirit) as well as appetite at the bottom. There are no continuums along these three lines, as Aristotle allows in his conception of the soul divisions into craft, scientific knowledge, prudence, wisdom, and understanding.
For Plato, the three castes are absolute. They can be described as castes, too, for Plato later applies the theory of the three-part soul to the theory of the three-part division of the polis or society. Plato suggests that a Virtuous and Just society is well-ordered, just as the Virtuous and Just person is well-ordered. In both a well-ordered person, Reason trumps everything else. Reason, and its expressions such as mathematical formulas and philosophical understanding, is the pinnacle of human existence. This is why a philosopher-king should always remain at the top of the social and political hierarchy. According to Plato, only a philosopher-king can rule with Justice and integrity. Those who are governed by lower-order soul qualities like energy and appetite are not fit for a Just leadership. The society will crumble under the leadership of those who are like the people Thrasymachus, Glaucon, and Adeimantus describe. Plato is not a realist; but his formula is tight. A well-ordered soul is a Just soul because the well-ordered soul fulfills its highest potential as a human being. The highest potential of a human being is the pursuit of Reason. Any person who is of the lower classes such as a craftsman or soldier is not fit for a position of leadership, because the craftsman (producer) and soldier are guided by passions that lead one away from the Truth. Plato's metaphysic is linked with his concept of Justice. Justice is not a human or lowly thing (such as a law and the person who obeys the law), but rather, a divine thing that Exists. Like the soul, Justice is real and must be strived for and achieved for Goodness and Happiness to occur. Aristotle allows…