Plato in his treatise on the ideal Republic identifies two parts of the soul as being opposite aspects. The first, “with which a man reasons, we may call the rational principle of the soul” and “the other, with which he loves and hungers and thirsts and feels the flutterings of any other desire, may be termed the irrational or appetitive…” [Republic IV 439]. The first aspect we generally call Reason. The second aspect has been called by many names, but may most easily be identified as Bodily Appetites.
The third aspect of the soul Plato identifies as “passion or spirit” [Republic IV 441]. This third aspect plays a role by aligning with one of the other two parts—either with reason or with bodily appetites. A useful designation for this part of the soul would be the Will. It is the driving force behind our actions. It weighs the desires against the reason, and motivates us in one direction or the other.
If the Will aligns with the appetitive part of the soul, then that man never rises above the animal state. Plato asks us to “observe whether a man is just and gentle, or rude and unsociable” in order to be able to distinguish which persons are ruled by either their reason or appetites. We do this naturally in the course of our social relations. We call people whose will aligns with their bodily appetites a selfish person. They are not good people. While we might not fully buy into Plato’s concept of a tri-partate soul, we still judge a person by their selfish or kindly actions just the same.
However you understand it, there are certainly at least two sides to our nature as humans. Whichever side you follow will determine whether you are living up to your fullest potential.