Plato had the conception that the soul was composed of three parts: reason, emotion, and desire/appetite. This is somewhat useful because it explains some of our ideas about how our conception of the self works. The rational area of the soul, which was the pinnacle, loves truth and wisdom. The emotional area of the soul loves honor and victory. Finally, the appetite area of the soul loves pleasure and money. When reason was operating correctly it had the virtue wisdom, emotions operating correctly had the virtue courage, and the appetites working correctly had the virtue temperance. As a result of three parts of the soul working correctly by achieving wisdom, courage, and temperance, the virtue justice would arise. Justice was a result of a healthy soul with each of its three parts working properly.
The Stoic conception of the soul is much more unified. The Stoics believed the soul or pneuma (breath) is an active material that was present throughout your passive material body, present in other organisms, in inanimate objects, and throughout the whole universe itself. The Stoics classified the pneuma as having four different types of tensile strengths. The most rarefied of pneuma was reason itself present only in humans. The least rarefied pneuma was present throughout the whole universe including humans.
The Stoics weren’t exactly panpsychists but they were “panpneumists.” They believe that an active airy/fiery breath was present throughout the cosmos and the most rarefied in the body of humans, specifically in the area of the brain (although they were once mistaken and thought reason was in the heart).
Plato’s tripartite theory of soul, as intuitive as it sounds, isn’t psychologically helpful. Plato had the idea that reason is a charioteer that steers two horses, a white horse which is emotion and a black horse which is desire. Unfortunately reason doesn’t exactly operate that way in my humble opinion. Reason doesn’t command emotions and desires, reason persuades emotions and desires by using therapy. The Stoics invented several techniques we can rationally use to persuade our emotions/desires and not have to compel them like a tyrant. If we try to compel our emotions/desires like a tyrant, they’ll push back.
In fact, the source of our negative emotions has a lot do with our reason itself making false judgments about externals. If we fix this issue by forming the correct judgments about externals our negative desires/sentiments will dissipate. Reason cannot remove negative passions until reason has fixed itself. Once you remove incorrect judgments from your rational faculty, your emotions will calm down and you’ll even feel some joy from this experience.