Full Idea
We have by nature these three appropriate relationships, corresponding to each form of the soul's parts - to pleasure because of the appetitive part, to success because of the competitive part, and to rectitude because of the rational part.
Clarification
'Rectitude' is 'uprightness', hence good behaviour
Gist of Idea
Each part of the soul has its virtue - pleasure for appetite, success for competition, and rectitude for reason
Source
Galen (On Hippocrates and Plato [c.170], 5.5.8)
Book Reference
'The Hellenistic Philosophers:Vol.1 translations', ed/tr. Long,A. /Sedley,D. [CUP 1987], p.415
A Reaction
This is a nice combination of Plato's tripartite theory of soul (in 'Republic') and Aristotle's derivation of virtues from functions. Presumably, though, reason should master the other two, and there is nothing in Galen's idea to explain this.