Plato, answered from the texts and cited to the page.
The soul has three parts — and the argument that establishes this begins from a principle no one can reasonably dispute: the same thing cannot do or undergo opposites in the same part of itself, in relation to the same thing, at the same time.1 If we ever find apparent opposites operating within a single soul simultaneously, we know we are not dealing with one thing but many.
The three parts are these. First, the logistikon — the rational part, that with which we learn, which is always straining toward truth and cares least for money and reputation.2 Second, the thumos — the spirited part, dedicated wholly to the pursuit of control, victory, and honor.3 Third, the epithumia — the appetitive part, named for the intensity of its desires for food, drink, sex, and the like; we also call it money-loving, since such appetites are most easily satisfied by means of money.4
That these are genuinely distinct parts, not one faculty wearing different masks, can be seen even in small children: spirit is present from birth, while rational calculation arrives late or not at all.5 Homer already understood this when he wrote of a man striking his chest and speaking to his heart — the part that has calculated about better and worse is plainly different from the part that rages without calculation.6
The spirited part deserves particular attention, because it is most easily mistaken for either reason or appetite. Consider what happens when a man believes he has been treated unjustly: spirit boils up, fights for what it takes to be right, endures cold and hunger in the cause — and will not rest until it prevails, or dies, or is called to heel by reason, as a dog is called by a shepherd.7
Spirit is reason's natural ally, not its rival; it is appetite that threatens to overturn the whole. Justice in the individual follows directly from this structure. Each part doing its own work — reason ruling and exercising foresight on behalf of the whole soul, spirit obeying and fighting as reason's ally, appetite governed and kept from insatiable expansion — that is what justice in a person is.8
Moderation is the friendly agreement among the parts that reason should rule; courage is spirit's preservation of reason's declarations about what is and is not to be feared through pleasure and pain alike.9 The virtues are not separate ornaments; they are names for the soul's proper internal order.
It is obvious that the same thing will not be willing to do or undergo opposites in the same part of itself, in relation to the same thing, at the same time. So, if we ever find this happening in the soul, we'll know that we aren't dealing with one thing but many.Complete Works, pp. 1096–1097
it is clear to everyone that the part with which we learn is always wholly straining to know where the truth lies and that, of the three parts, it cares least for money and reputation.Complete Works, pp. 1217–1218
What about the spirited part? Don't we say that it is wholly dedicated to the pursuit of control, victory, and high repute?Complete Works, pp. 1217–1218
we named it after the biggest and strongest thing in it. Hence we called it the appetitive part, because of the intensity of its appetites for food, drink, sex, and all the things associated with them, but we also called it the money-loving part, because such appetites are most easily satisfied by means of money.Complete Works, pp. 1217–1218
Even in small children, one can see that they are full of spirit right from birth, while as far as rational calculation is concerned, some never seem to get a share of it, while the majority do so quite late.Complete Works, pp. 1101–1102
Homer clearly represents the part that has calculated about better and worse as different from the part that is angry without calculation.Complete Works, pp. 1101–1102
if, instead, he believes that someone has been unjust to him? Isn't the spirit within him boiling and angry, fighting for what he believes to be just? Won't it endure hunger, cold, and the like and keep on till it is victorious, not ceasing from noble actions until it either wins, dies, or calms down, called to heel by the reason within him, like a dog by a shepherd?Complete Works, pp. 1101–1102
each one of us in whom each part is doing its own work will himself be just and do his own... isn't it appropriate for the rational part to rule, since it is really wise and exercises foresight on behalf of the whole soul, and for the spirited part to obey it and be its ally?Complete Works, pp. 1102–1103
a person will be just because of what we've so often mentioned... And isn't he moderate because of the friendly and harmonious relations between these same parts, namely, when the ruler and the ruled believe in common that the rational part should rule and don't engage in civil war against it?Complete Works, pp. 1102–1103