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Plato's Theory of Justice: An Analysis

An Evaluation of Plato's Three Definitions of Justice

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By speaking through Socrates, Plato strives to answer two fundamental questions of justice: what it is, and why it is essential in society. Book One of Plato’s Republic presents three suggestions as to how justice should be defined, offered by Cephalus, Polemarchus and Thrasymachus. I will discuss and emulate the suggestions using Socrates’ argumentation. I will also explain how all three definitions relate to one another. By using and closely reading Book One, the way in which these perspectives shape Socrates’ quest will be evident in that he must not only unravel the meaning of justice but also its validity in social norms.

Book One begins with Socrates and Cephalus discussing “the threshold of old age” and what wealth means to a man (Plato 5). Cephalus believes that wealth is not necessarily the cause of one’s happiness; rather, it allows him or her to live a moral life. This immediately triggers a debate about justice. Cephalus introduces his view: he believes that justice is about acting in accordance with one’s legal responsibilities by returning debts and always demonstrating trustworthiness (Plato 7). He says, “wealth can do a lot to save us from having to cheat or deceive someone against our will from having to depart for that other place in fear because we sacrifice to a good or money to a person,” which explains that people who are financially stable are more likely to keep in good terms with creditors than those who are more inclined to lie or steal to avoid situations where they cannot return their debts (Plato 7). 

Socrates argues Cephalus’ opinion; depending on the circumstances, it may be unjust to always tell the truth or return debts. He justifies his reasoning by using an example of lending a weapon: it would be unjust for a person to return a borrowed weapon to a madman, even if the debtor is fulfilling his or her legal duties, because the weapon is threatening to society when in the hands of a madman. (Plato 7). Generally speaking, returning belongings to those who should not possess them is unjust, as they could potentially endanger lives. As well, the debtor should not speak the entire truth to someone who is insane. An insane person usually neglects or denies what they are told, especially if it is a commentary on their wrongdoing. Socrates concludes that Cephalus’ definition is inadequate (Plato 7).

Polemarchus agrees with Cephalus’ definition, as it is based on Simonides’ rule of justice: “it is just to give to each what is owed” (Plato 7). He adjusts this rule to create his own definition: it is just to give to people what they are entitled to; however, it is unjust to give what is harmful to friends. To summarise, Polemarchus asserts that justice is about helping friends and harming enemies (Plato 8). In response to Socrates, who asks how can a just man best help friends and harm enemies, he answers that, in his opinion, “it is in making war and being an ally in battle” (Plato 8). This justification supports the just man’s intentions in attacking enemies and assisting friends in battle, which promotes reliance among friends and knowing clearly who his friends and enemies are. 

This does not always work, because if trust is broken among allies then they will turn on each other instead of their opposition. There is also a conflict with one ally affiliating with both another ally and his or her enemy, in which the first ally is torn between them. Socrates uses four cases against this claim, with the first two having negative conclusions and the latter two having positive ones. First, Socrates affirms that a just person is useless in peacetime, because that person is only useful when there is no conflict in order to maintain peace (Plato 10). The next case can be perceived in two ways: either the person is both just and unjust, or justice itself is morally neutral. This means that the person will do bad or good to both friends and enemies, depending on the treatment they deserve for their actions (Plato 11). 

The third case contradicts the second; it maintains that the just individual must know his or her friends and enemies. Making that distinction signifies that the individual must always help friends and harm enemies no matter what (Plato 12). Lastly, a just being harms no one. Regardless of the relationship between the two, a just individual always treats another person fairly. Socrates rationalises that one should not combat evil with evil because it is unjust to harm anyone, and to harm is the role of the unjust individual (Plato 13).

When Polemarchus agrees with Socrates’ argument, Thrasymachus demands that Socrates proposes his definition. Socrates admits that he cannot provide the exact definition and wants to pursue it (Plato 14). Thrasymachus attempts to conclusively define justice; he explains that it is the advantage of the stronger (Plato 17). To elaborate, he is saying that justice seeks the best interests for the stronger party in society. Justice is obeying the law, and the ruler, being the stronger, creates laws based on those interests, and the weaker party is obliged to follow them (Plato 16). Simply put, in order to be just, one should be legally bound (Plato 17). 

This is the point where the argument shifts from defining justice to determining whether justice or injustice is better, as the question becomes whether or not an individual is just for the sake of others (Plato 23). Socrates declares that justice is more reputable because even among the most unjust groups of people, there needs to be a degree of loyalty, such as with thieves. This rule applies to the individual, because everything he or she does for society should be done well in pursuance of profitability and happiness (Plato 34). In other words, justice consists of wisdom and goodness, whereas injustice consists of ignorance and badness (Plato 33).

Despite the distinction between these definitions, they share common elements that contribute to Socrates’ learning and the development towards a precise definition. Looking at the first two propositions offered by Cephalus and Polemarchus, they are similar in the sense that although giving what is deserved or returning something owed to a friend and always telling him or her the truth is a moral obligation, the just individual must also give back to an enemy and show candidness towards that person in respect to whatever wrongdoing he or she has committed towards the individual. 

Taking Polemarchus’ account and comparing it with Thrasymachus’, they exhibit similarities in that justice can only be achieved by the stronger if allies join together and form the stronger party in common interest, and eliminate those who are against the wishes and expectations of the party. Cephalus and Thrasymachus’ ideas correlate, as the stronger will be well off by being just, in that they will never have financial issues if debts are always paid off and if they show honesty in every matter concerning monetary value. All three definitions express a theme, which is removing the distinction between profit and justice.

In spite of Socrates’ inability to generate a universal definition of justice at this stage, the three proposals discussed and the interrelatedness between them have nevertheless set the blueprint for Plato’s subsequent Books in The Republic associated with developing a definition and ground rules for justice. The accounts talk about paying back debts and being honest, helping friends and harming enemies, and pursuing profitability for the stronger. Though they are flawed, the argument for returning debts and honesty is best because it is a popular convention in banking, mortgages, and any activity involving rentals, among other dealings involving money and debts. The main concern now is how Socrates will go about taking the conversation to an extensive level using these initial accounts provided and his critique of them, in order to describe what justice truly entails.


Plato. The Republic of Plato: Transl. with Notes and an Interpretative Essay by Allan Bloom. Trans. Allan David Bloom. New York: Basic, 1991. Print.

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