Plato's tripartite theory of soul
Plato's tripartite theory of soul is a theory of psyche (psychology) proposed by the ancient Greek philosopher Plato in his treatise the Republic. In it, Plato argues that the soul is composed of three parts: The logical (λόγος), the spirited (θυμός) and the appetitive (ἐπιθυμητής). These three parts of the soul also correspond to the three classes of a just society. Individual justice consists in maintaining these three parts in the correct hierarchy, where the logical part (aided by spirited) rules, and the appetitive obeys.
In Book IV of the Republic Socrates and his interlocutors are attempting to answer whether the soul is one or made of parts. Socrates states that, "It is clear that the same thing will never do or undergo opposite things in the same part of it and towards the same thing at the same time; so if we find this happening, we shall know it was not one thing but more than one."  (This is an example of Plato's Principle of Non-Contradiction). For instance, it seems that, given each person has only one soul, it should be impossible for a person to simultaneously desire something yet also at that very moment be averse to the same thing, as when one is tempted to commit a crime but also averse to it. Both Socrates and Glaucon agree that it should not be possible for the soul to at the same time both be in one state and its opposite. From this it follows that there must be at least two aspects to soul.
The logistikon (from Logos) is the thinking part of the soul which loves the truth and seeks to learn it. Socrates originally identifies the soul dominated by this part with the Athenian temperament. The logistikon discerns what is real and not merely apparent, judges what is true and what is false and wisely makes just decisions in accordance with its love for goodness.
Socrates makes the point that the logistikon would be the smallest part of the soul (as the guardians would be the smallest population within the Republic) but nevertheless a soul can only be declared just if all three parts are in agreement that the logistikon should rule.
According to Socrates, the spirited (θυμός) is the part of the soul by which we are angry or get into a temper. He also calls this part 'high spirit' and initially identifies the soul dominated by this part with the Thracians, Scythians and the people of 'northern regions.'  In the just soul, the spirited aligns with the logistikon and resists the desires of the appetitive, becoming manifested as 'indignation' and in general the courage to be good. In the unjust soul, the spirited ignores the logistikon and aligns with the desires of the appetitive, manifesting as the demand for the pleasures of the body.
The appetitive is the part of the soul by which we experience carnal erotic love, hunger, thirst and in general the desires opposed to the logistikon. (The appetitive is in fact labelled as being 'a-logical') Socrates also identifies this part of the soul with the pleasure involved in human reproduction. He further relates this part to the love of money-making, which he mentions as being the particular mark of the Phoenicians and Egyptians.
- "Plato's Ethics and Politics in The Republic" at the Stanford Encyclopedia of Philosophy Retrieved August 29, 2009
- Republic IV: 436 B6 - C1 (W.H.D. Rouse translation)
- "Ancient Theories of Soul" at the Stanford Encyclopedia of Philosophy Retrieved August 29, 2009
- Republic IV 435 E8-9
- Republic IV 442A
- Republic IV 439 E3-4
- Republic IV 435 E4-8
- Republic 439 D5-7
- Republic 439 D7
- Republic 436 A1-3
- "Plato" at the Internet Encyclopedia of Philosophy
- "Plato" at the Stanford Encyclopedia of Philosophy