Plato endeavored to systematically explain inner conflict and psychological complexity. The approach he takes is to categorize three parts of the ‘soul’ (which can be taken to mean mind or psychology). These parts are reason, emotion and desire. He is able to argue firstly for the existence of a complex set of relations between various aspects of human psychology through the proposition that one thing cannot simultaneously either act or be acted on in opposite ways in the same respect and in the same context. He goes on to illustrate that whilst a man can be thirsty, he can also prevent himself from drinking, thus concluding that there are two parts that act in opposite ways within the mind. These parts are reason and desire, correlating to rationality and irrationality respectively. Passion is next identified as a third part that may be interchangeable with either of the other two.
The contrast between reason and desire is more readily observable than that of emotion and each of the others. Reason and desire are the cause of duality in man. Reason correlates with virtue and morality (considered choices, discipline, self-control, and charity) whereas desire correlates with immorality (immediate gratification of the senses, vulgarity and lack of forethought). Desire is the instinctual and irrational side of the human character. Reason is concerned with the overall good of the person as a whole (note: not all reasoning has to be good reasoning).
Emotion and reason have a relationship whereby emotion is subordinate to reason and operates as a function of rationality. Similar to desire, emotion responds to the object of impulse without thought or a system of regard for the overall good of the person as a whole. This response however aligns itself with reason in order to act as a vehicle of motivation toward the ideal proposed by reason.
Emotion is aligned with reason as a force that generally conflicts with desire. One example might be a man who curses himself for letting his desires compel him to act against his reason. Another might be the experience of feeling wronged by another, in which an emotion such as anger is aligned with ones reasoning as to what is right. Additionally, such an emotion that is tied to the feeling of being wronged can compel you to withstand or subdue desires, such as withstanding hunger or cold (with examples such as these Plato is using the word ‘passion’ in place of emotion). Emotion therefore acts on behalf of reason in opposition to desire even if it does not understand the ideal that this particular instance of reasoning aspires to.
Plato also proposes a metaphorical and theoretical model of the mind in which reason, desire and emotion are represented by the image of a man, a many-headed mythological beast and a lion respectively. The whims of the beast and the lion correlate with immorality, and the restraint and control of the man with morality. Plato is arguing that you (a person) is made up of all three elements, though in order reach an ideal state of being, reason must subdue first emotion, in order that emotion be employed to subdue desire.
(Julia Annas: Voices of Ancient Philosophy – Oxford University Press 2001)