Plato's - The Republic
In his The Republic Plato writes about dialogues between Socrates and others concerning Justice, and the establishment of an Ideal State where Justice should prevail.
Having defined Justice as "the having and doing of what is one's own" and effectively suggesting that "a just man is a man in just the right place, doing his best, and giving the full equivalent of what he receives" the dialogue moves toward asserting that an Ideal State will necessarily be composed of three classes of persons because of the existence of the three parts of the soul: and that one part tends to 'prevail' in each individual person.
Plato holds that Human Psyche or Soul of each individual is "Tripartite" and moves on to categorise three classes of people differentiable by which of - appetite, spirit or reason - prevails in their own individual souls.
In Book 4 of Plato's The Republic there is a passage which initially suggests the three parts of the soul tripartism which forms the basis for Plato's later asserting that an Ideal State should be peopled by three classes of persons:
...can we possibly refuse to admit that there exist in each
of us the same generic parts and characteristics as are found in
the state? For I presume the state has not received them from any
other source. It would be ridiculous to imagine that the presence
of the spirited element in cities is not to be traced to
individuals, wherever this character is imputed to the people, as
it is to the natives of Thrace, and Scythia, and generally
speaking, of the northern countries; or the love of knowledge,
which would be chiefly attributed to our own country; or the love
of riches, which people would especially connect with the
Phoenicians and the Egyptians.
Certainly.
This then is a fact so far, and one which it is not difficult to apprehend.
No, it is not.
But here begins a difficulty. Are all our actions alike performed by the one predominant faculty, or are there three faculties operating severally in our different actions? Do we learn with one internal faculty, and become angry with another, and with a third feel desire for all the pleasures connected with eating and drinking, and the propagation of the species; or upon every impulse to action, do we perform these several actions with the whole soul…Socrates / Socratic Dialogue suggesting the existence of three parts of the soul
Certainly.
This then is a fact so far, and one which it is not difficult to apprehend.
No, it is not.
But here begins a difficulty. Are all our actions alike performed by the one predominant faculty, or are there three faculties operating severally in our different actions? Do we learn with one internal faculty, and become angry with another, and with a third feel desire for all the pleasures connected with eating and drinking, and the propagation of the species; or upon every impulse to action, do we perform these several actions with the whole soul…
Socrates / Socratic Dialogue suggesting the existence of three parts of the soul
from Plato's Republic Book 4
Plato further suggests that persons in whom appetite prevails should become Artisans and producers, persons in whom spirit prevails should become Auxiliaries and be employed in defending the state against external threats and internal disorders, and persons in whom reason prevails should undergo, rigorous processes of selection, followed by very long periods of training towards better preparing them to become philosopher-rulers.
Hence Plato's identification of the three classes of persons who will people his Ideal State arises out of what he believes to be the three parts of the soul or "tripartite" nature of the Human Psyche or Soul.
Peopled by three classes of persons - Artisans, Auxiliaries and Philosopher-rulers, a state could ideally hope for Justice to prevail where each class of person fulfilled their proper function as producers, defenders and rulers and did not interfere with each others' fulfillment of their individually necessary contributions to the functioning of the state.
Several truly notable authorities
endorse Tripartite Soul Theory
Key Socratic Dialogues from
Book 4 and Book 9 of Plato's Republic
Plato's Ideal State Plato's Chariot allegory
Philosophy - Eastern and Western & 'Tripartite' Human Nature
FIVE major World Religions & 'Tripartite' Human Nature