P15: A Desire which arises from a true knowledge of good and evil can be extinguished or restrained by many other Desires which arise from affects by which we are tormented.

Dem.: From a true knowledge of good and evil, insofar as this is an affect (by P8), there necessarily arises a Desire (by Def. Aff. I), which is the greater as the affect from which it arises is greater (by IIIP37). But because this Desire arises (by hypothesis) from the fact that we understand something truly, it follows in us insofar as we act (by IIIP3).10 And so it must be understood through our essence alone (by IIID2), and consequently (by IIIP7), its force and growth can be defined only by human power alone.

Next, Desires which arise from affects by which we are torn are also greater as these affects are more violent. And so their force and growth (by P5) must be defined by the power of external causes, which, if it were compared with ours, would indefinitely surpass our power (by P3). Hence, Desires which arise from such affects can be more violent than that which arises from a true knowledge of good and evil, and can therefore (by P7) restrain or extinguish it, q.e.d.