and applying it to the understanding, he shows how it is reduced from potential to act, not indeed by suffering, but rather by perfecting itself

 He distinguishes that every one is in power bifurcated; either simply, or in some sense; and applying it to the understanding, he shows how it is reduced from potential to act, not indeed by suffering, but rather by perfecting itself

https://aquinas.cc/la/en/~DeAn.Bk2.L10.v417a17.27