Full Idea
A power is not the cause of an operation, but only the cause's aptitude for operating.
Gist of Idea
A power is not a cause, but an aptitude for a cause
Source
Jacob Zabarella (De rebus naturalibus [1590], De fac anim 4:col 692), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 23.5
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.538
A Reaction
His example is the power of running, which is actually caused by the soul (or whatever), which generates the power. A power is a very superficial thing.