What is Act and Potency?
In the simplest terms, act and potency are principles of being in which act is “that which is,” and potency is “that which can be.” More specifically, act is that by which a being (1) is; (2) is some kind of thing; and (3) exists according to some modification. Act as esse (that by which a being “is”) is act without qualification, while other acts (that by which a being is some kind of being and exists according to some modification) are acts by analogy. In contrast, potency is that by which a being (1) can be; (2) can be some kind of thing; and (3) can exist according to some modification. Primary matter (that by which a being can be) is pure potency without qualification. All other potencies (that by which a being can be some kind of thing and can exist according to some modification) are potencies by analogy. To illustrate, act is to potency as seeing is to the inactivity of closed eyes, as a ball rolling is compared to it being at rest, or as a candle thus molded is to a lump of wax being capable of being thus molded.
The beings of our experience are not pure act nor pure potency, but without exception are limited beings composed of act and potency. Several observations about the relationship of act and potency can be made. First, act and potency stand in correlative opposition to one another. Act is the act of some potency; every potency is the potency to some act (Theorem I). Second, act and potency are not beings, but principles of being. If they were both beings, they could not be found combined within a single being, since one being plus one being equals two beings, not one (Theorem II). Third, act is unlimited in itself, and only limited by the potency in which it is received (Theorem III). Fourth, while act can be known by itself, potency is known only through act (Theorem IV). Finally, act and potency divide being in the orders of existence, essence, substance, and operation (Theorem V). In the order of existence, composed beings are divided by esse (act) and essence (potency). Act is what makes an essence to be, but essence limits act to this particular act. In the order of essence, being is divided by substance (act) and accidents (potency). In the order of substance, being is divided by substantial form (act) and primary matter (potency). In the order of operation, being is divided by the activity itself (operation) and the potency to act (power).
The Right Way to Talk About Change
To “change” is to “become other than what one was.” This seemingly obvious definition, however, poses a problem for the essentialist who asserts that for a thing to be real is to be the kind of thing that it is. For if “to be” is “to be a what,” then how can something become other than what it is? Parmenides attempted to answer by denying the reality of change. Beginning from an essentialist view of reality, Parmenides argued that if the real were to change, it would have to be what-it-is and at the same time what-it-is-not. But because this is an obvious contradiction, change is therefore an illusion. Another attempt at solving the problem came from Heraclitus, who, though beginning from the same essentialist viewpoint, arrived at the opposite conclusion from Parmenides, affirming change and denying any permanence in being.
These “solutions” offered by Parmenides and Heraclitus illustrate how an essentialist metaphysic that views reality as that-which-is-what-it-is has great difficulty explaining how things can change. There is, however, a way to talk about being and change that does not have to deny either change or permanence, and that is by the principles of act and potency. When we say that a seed changes into a tree, we are implying that there is an identity (sameness) and a lack of identity (distinction) throughout the process. In other words, while change means that something is different about the thing, it also implies that something remains the same. If there was no identity, then it would make no sense to talk about “the seed changing into the tree” because in this case the seed and the tree would be completely distinct beings, leading to the extreme of Heraclitus. Moreover, if there was no distinction, then the “change” from seed to tree is a mere illusion. This, of course, is the answer that Parmenides offered. But not only is it natural to speak about “the seed changing into the tree,” it is also quite evident to the senses and the intellect that this is exactly what occurs.
The principles of act and potency solve the problem in this manner. At the initial stage, the seed is “being in act” as regards the characteristics it has before the change. That is, it actually possesses the perfections of a seed. But the seed is also “being in potency” as regards the characteristics it has after the change, at the terminal stage. Though as a seed it is not a tree, it has the potency for acquiring that perfection. At the terminal stage, the being (now the “tree”) is “being in act” as regards its present characteristics; its potency has been actualized. Throughout the process, however, there is a common subject, that which gives the being its identity through change. This common subject is not considered as determined by the characteristics of either the initial or the terminal stage, but that which is common to both, a subject that undergoes the process of change. The same being, by virtue of act and potency, can thereby undergo change while retaining identity. Thus, change and being are compatible, and unlike Parmenides and Heraclitus, we need not retain one while attempting to explain away the other.