Dr. Ronnie Klein
Glaucon and Adeimantus' challenges justice by way of three views, these views are 1. Glaucon's social contract theory (358e-359b), 2. The story of Gyges' Ring (359b-360e), and 3. The comparison of lives (360e-362c). Adeimantus says that according to the most common defenses of justice that are offered, the advantages of justice come from seeming just, not from being just (362e-367a). In other words people are only "just" because of the penalties that follow, if being "unjust" had no penalties then surely everyone would be "unjust". Socrates says in order to answer this challenge; he needs to know what justice is. Socrates explains that it will be easier if we talk about justice in the city as a whole rather then the individual 368d-e. Plato or Socrates answers the question via three parts - 1. Plato presents a division of the soul into 3 parts - the appetitive part, the spirited part and the rational part (436a-441b), 2.
Plato gives an explanation of the virtues in terms of these 3 parts of the soul, similar to his account of the virtues of the city (441c-444b) and 3. Plato uses this account of the virtues to explain very vaguely why the virtuous person is always happier than the vicious person (444c-445c).
Plato's first part of the soul is the appetite, which includes all our myriad desires for various pleasures, comforts, physical satisfactions, and bodily ease. There are so many of these appetites that Plato didn't bother to mention, but he does note that they can often be in conflict even with each other. The second is the spirited, or hot-blooded, part, i.e., the part that gets angry when it perceives, i.e. an injustice being done. This is the part of us that loves...