Plato's tripartite theory of soul

Plato's tripartite theory of soul is a theory of psyche proposed by the ancient Greek philosopher Plato in his treatise the Republic. In Republic, Plato asserted that the (psyche) is composed of three parts; the (logistikon, logical), the θυμοειδές (thymoeides, spirited) and the ἐπιθυμητικόν (epithymetikon, appetitive). These three parts of the ψυχή also correspond to the three classes of a society. Whether in a city or an individual, (dikaiosyne, justice) is declared to be the state of the whole in which each part fulfills its function without attempting to interfere in the functions of others. The function of the ἐπιθυμητικόν is to produce and seek pleasure. The function of the λογιστικός is to gently rule through the love of learning. The function of the θυμοειδές is to obey the directions of the λογιστικός while ferociously defending the whole from external invasion and internal disorder. Whether in a city or an individual, (adikia, injustice) is the contrary state of the whole, often taking the specific form in which the spirited listens instead to the appetitive, while they together either ignore the logical entirely or employ it in their pursuits of pleasure.

In Book IV of the Republic Plato and his interlocutors are attempting to answer whether the soul is one or made of parts. Plato 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 Plato 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 logical or logistikon (from logos) is the thinking part of the soul which loves the truth and seeks to learn it. Plato originally identifies the soul dominated by this part with the Athenian temperament. The logistikon discerns what is the 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.

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