The first meaning of “potency” that Aristotle gives names a kind of act. In fact, the whole doctrine of potency and act is a development of that first notion of “dunamis” or “potentia”, which is closest to the English word “power”.
What is power? The ability to cause change in another. The account of the word has been uniform from Aristotle to contemporary physics. This ability is either simply present, or present and exercised, and if present it is either present simply as a power, or as the source of power. This gives us three notes to power:
1.) Its principle (nature)
2.) The very power to cause (power)
3.) The very power to cause, as causing (operation)
All three notes in the word “power” are acts. The second meaning is the first sense of “power” and the other two are inseparable from it. Meanings 2 and 1 of the word “power” are called “first act” by the Scholastics, and meaning 3 is called “second act”. Missing this will make much of Aristotle’s thought seem howlingly stupid: later-developed axioms like “everything acts so far as it is in act” and “everything in potency is brought into act by something in act” or “every cause gives its own act to its effect” (or any axiom about causality at all) are obviously false unless one recognizes that power is act. Anthony Kenny, for example, wonders how St. Thomas could every say something as stupid as:
But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.
What simplemindedness! It is obvious that one can cause fire by rubbing two sticks together! The joke is on Kenny, though, since power is act. Good luck starting a fire with two sticks (or anything else) that do not have the power to start one. This is not some arcane or hidden meaning of “potentia”, knowable only after many subtle inferences: St. Thomas wrote a whole book called “de potentia” which begins with an explanation of how potentia is a kind of act.
The notion of power is the door to understanding both act and potency. Power itself first describes act, and comes to also describe potency. Potency in the sense of passive potency first arises not as its own independent notion, but as that which power acts upon. It is “that which looks to power” and depends or is changed by it. So considered, there is passive power in anything which can be changed by another or anything which depends on another. On this account, “pure potency” is simply the ability to be changed by another into a purely different being. Potency is a kind of dependence or ability to be worked upon, and so it is utterly distinct in being from power as such.
Just Thomism 