When the intellect regularly grasps its proper object--truth--it becomes habituated to beholding the truth. The mind becomes stamped by the truth, much like the metal of a coin is stamped with the head of a sovereign. This quality of being formed by the truth is good for the intellect.

[The will's] proper object is the authentic good. When the will continually chooses what is truly good, it becomes stronger, more energetic, more stable, more excellent in its choices and loves. Likewise, when the will rejects what is evil and shuns the corruption of immorality, it comes to love and appreciate the good even more...

... the vast difference between matter and spirit entails that they can operate somewhat independently within a single person.

Aquinas notes that we experience how ... emotional movements fight against reason "to the extent that we sense or imagine something pleasurable that reason forbids, or something unpleasant that reason commands."

If any movement of the body arose independently of deliberate choice [before the fall], that movement was nevertheless good, for it was guided by the grace that always ordered the entire person to the ultimate good. When Adam and Eve sinned, however, they lost this grace for themselves and for the rest of us. The first sour note in harmonious nature was sounded by original sin. The body was no longer ordered to the rational good but to the sensory good; it was no longer directed to heaven but to earth.

Reason at the service of the irascible appetites depicts oneself as a righteous crusader for justice when, in actuality, one becomes a knight in Satan's service.

What does it mean to tame the emotions? In a phrase, it means to habituate them to right reason's good influence.