The act-potency distinction is a key orientating principle of Thomistic metaphysics (if not the orientating aspect)—having implications and applications for issues of substance and accidents, essence and existence, as well as causation. It has been noted that “[the act-potency distinction] is absolutely crucial to the Scholastic approach to the questions about the…philosophy of nature, philosophical psychology, natural theology, and even ethics.”  While the act-potency distinction has a multitude of applications for other metaphysical considerations and constructive uses , we will restrict ourselves here to considering the distinction in itself as much as possible, without reference to other metaphysical or theological considerations except as examples for clarity.
The historical origin of the act-potency distinction is found in ancient philosophy: specifically, on the issues of being versus becoming (i.e., permanence and change) and Aristotle’s response to it. The two main figures representing views of change are Parmenides and Heraclitus.  Parmenides argued from the principle of contradiction that all becoming (i.e., change in a being) is impossible, since it would require being to arise out of non-being, which is impossible since, as the axiom goes, ex nihilo nihil fit (Lt. ‘out of nothing, nothing comes’). As Parmenides himself says:
“Being is ungenerated and imperishable, whole, unique, immovable, and complete…. For, what origin could your search out for it? How and when did it grow? Not from non-Being shall I allow you to say or to think, for it is not possible to say or to think that it is not…. Thus it is necessary either to exist all in all or not at all…. How could [Being] have come into being?… So coming into being is extinguished and perishing is unheard of.” 
Edward Feser helpfully outlines this argument in syllogistic form:
(1) Change would require being to arise out of non-being or nothingness.
(2) But from non-being or nothingness, nothing can arise.
(3) So that change is impossible. 
Opposite to Parmenides is the position of Heraclitus, who asserted that all that there is, is becoming – that is, being (as Parmenides conceives of it) does not exist, rather all is pure becoming and “the opposition of being to non-being is a [“purely abstract”] opposition.”  For Heraclitus, becoming swallows being entirely so that there is nothing constant in any thing but all is in constant change, or constant flux. Everything changes and nothing remains the same; his famous illustration is that you cannot step in the same river twice, because the particulars which made up that river have changed since you last stepped in it and thus, it is no longer that river but this river.
Situating himself between these two philosophies of being Aristotle asks, “Are we really forced to choose between change and being?”  Aristotle presents us with a distinction between act and potency, or being-in-act and being-in-potency which resolves the problem. This re-orientates the discussion about being and becoming: change is not, then, being arising from nonbeing, as Parmenides wrongly assumed it must be, since, as he rightly assumes, ex nihilo nihil fit. Aristotle does not take issue with the second premise in Parmenides’ syllogism, rather it is with the first premise that he dissents. For Aristotle (and Thomas), change is a process wherein one kind of being (e.g., being-in-act) arises from another kind of being (e.g., being-in-potency), or “to become in act what was in potency.”  This view which Aristotle presents is truly a middle way: it does not deny the reality of being nor of becoming but rather seeks to hold these two together as differing aspects of reality, understood in relation to one another as distinct.
In this way, then, act and potency are commonly understood to be “correlative intelligibilities” – that is, they are properly understood only “in relation to each other.” Such that, “act is the act of a potency, potency is the potency to some act.”  A being is what it is because it existed with prior potential. Even the nature of existence is understood in these terms. God’s own idea of His creation is it’s essence, which he brings into actual existence in time and space.
Now, what is actuality? And what is potentiality? These are both states of being. This does not refer to abstract principles by which a thing acts, but rather real conditions of particular beings. Here I am distinguishing act from actuality, or being-in-act, and potency from potentiality, or being-in-potency; act and potency are called principles of being insofar as they are abstracted from particulars, while being-in-act and being-in-potencies are states of being since they are not abstracted from their particular instances. Therefore, actuality, as a state of being, only exists in a being insofar as that being is in act with regard to a particular attribute; similarly, potentiality, as a state of being, only exists in a being insofar as that being is in potency with regard to a perfection. While actuality and potentiality are both states of being, there remains a difference between them. This is shown by their definitions:
Actuality is a state of being (i.e., being-in-act) wherein a perfection is realized, or actual; potentiality is a state of being (being-in-potency) wherein there is a capacity to receive a perfection, or act.
Actuality refers to those aspects of a being which are actual, such as existence; while potentiality refers to those aspects of a being which are possible to be actualized—it is a capacity for further actualization. This is the simplest expression of the act-potency distinction. From the definitions given above there are several points that must be explicated for a better grasp of the act-potency distinction.
1. Being is distinguished into two types, or aspects: actuality and potentiality.
It is necessary to understand that by actuality and potentiality is meant states of being; this entails that being, in the abstract, is comprised of two intrinsic principles: act and potency. These principles of being (i.e., act and potency) are the abstract corollaries to the concrete states of being referred to by actuality and potentiality—commonly also referred to as being–in–act and being–in–potency.  An implication for this point is that neither act nor potency are more properly applied to being – either in the abstract or the concrete; rather, these two are two aspects of being which are properly distinct. Simply put, being-in-act and being-in-potency are both of positive metaphysical value, whereas nonbeing has no metaphysical value (since nonbeing is, properly speaking, nothing).
2. Being, in-act and in-potency, is opposed to nonbeing.
A common misunderstanding of the former point results in setting act and potency as opposed to one another entirely, as though they are on two opposite poles wherein act equates to being and potency to nonbeing.  While this language may be interpreted to mean something close to the truth of the matter (e.g., that act and potency are two opposed aspects of being, insofar as act is not potency and potency is not act), it generally demonstrates shallow thinking about act and potency. It is necessary, in remaining consistent with the previous point, to assert that being, -in-act and -in-potency, is opposite to nonbeing. Being-in-act and being-in-potency are opposite to nonbeing. This is based upon what is called the contrariety of concepts—being (defined loosely as that which is—i.e., that which has positive metaphysical value) is, by definition, opposed to nonbeing (defined as that which is not, or nothing—i.e., that which has no positive metaphysical value). This principle of the contrariety of concepts is also useful for the distinction between actuality and potentiality, insofar as the former is that which is actual in a being and the latter is that which is possible in a being—these are, by definition, contrary things (e.g., a being cannot be simultaneously in act and in potency with regard to the same perfection).
3. Actuality is prior to potentiality in nature, excellence, and intelligibility. (Despite their being correlative intelligibilities.)
This is the most complicated of the three principles of relation between actuality and potentiality. Here ‘prior’ takes upon no overtly technical meaning: it simply means that actuality is before potentiality—in both the order of being and knowledge. Allow me to break this principle into three sub-principles for ease of understanding. 1.) Actuality is prior to potentiality in nature. By this is meant that potentiality, insofar as it is the potency of a being, cannot exist by itself or apart from a composition with act.  For any potentiality is predicated upon a being’s possession of requisite actuality—at the very least the act of existence/esse—since, “a thing’s potencies are grounded in its actualities.”  Actuality, on the other hand, is not metaphysically dependent upon potentiality but rather is limited by its composition with potentiality. For in any finite being its actuality, while united to potentiality, is not dependent upon that potentiality in order to be. 2.) Actuality is prior to potentiality in excellence. This is an implication of the previous sub-principle: if something is prior in nature, it is also prior in excellence. Actuality, insofar as it is not dependent upon potentiality, is more excellent than potency, which is dependent upon actuality. This is evident even with the briefest of examinations: do we a praise a thing for its capacity to possess further perfections on account of the capacity alone or on account of it being a capacity for perfection? Clearly it is the latter, for perfection is better than capacity—even capacity for perfection. 3.) Actuality is prior to potentiality in intelligibility. This a further extrapolation from the initial sub-principle. For insofar as actuality, as a state of being, is prior to potentiality in nature so it is known first, since any potentiality is known only with reference to requisite actuality, while that requisite actuality is known only with reference to itself. Therefore, it is prior in intelligibility.
Now the astute reader will note that this seems to go against what I have said previously. I said that act and potency were “correlative intelligibilities,” and only understood in relation to each other, since, “act is the act of a potency, potency is the potency to some act.”  But now I am saying that actuality is prior in intelligibility to potentiality. Is this a contradiction? No. Here is why: I have also previously distinguished between “intrinsic principles of being” and “states of being”—the former refers to the abstraction of principles from the particulars to attain knowledge of the things (in this case act and potency) in themselves, while the latter refers to the concrete particulars themselves. In the abstract, act and potency are correlative intelligibilities, since the concepts are mutually independent; however, in the concrete, actuality and potentiality (or being-in-act and being-in-potency) are not mutually dependent with regard to intelligibility but rather one is known with reference to the other while that other is known only with reference to itself. As a state of being, actuality is that which is actual, while potentiality is the potentiality of an actuality.
In conclusion, the act-potency distinction, while basic to understanding Thomistic metaphysics, is not easy to grasp nor is it necessarily a “simple” distinction. I hope this post was helpful to you, my readers.
— Michael Hall
 Edward Feser, Scholastic Metaphysics: A Contemporary Introduction (Piscataway, NJ: Transaction Books, 2014), 31.
 For constructive uses, see Michael Hall, Act and Potency as Division of Being: A Philosophical Theology of the Creator-creature Distinction (where the act-potency distinction is used to argue for a qualitative Creator-creature distinction, as opposed to a quantitative distinction which is implicit in many discussions of theology proper today)..
 For overviews of Parmenides see John Palmer, “Parmenides,” in The Stanford Encyclopedia of Philosophy (Winter 2016 Ed.), ed., Edward N. Zalta, . For an overview of Heraclitus see Daniel W. Graham, “Heraclitus,” in The Stanford Encyclopedia of Philosophy (Fall 2015 Ed.), ed., Edward N. Zalta, .
 Leonardo Taran, ed. and trans., Parmenides: A Text with Translation, Commentary, and Critical Essays (Princeton: Princeton University Press, 1965), 85; cf. Kathleen Freeman, The Pre-Socratic Philosophers: A Companion to Diels, Fragmente der Vorsokratiker (Oxford: Basil Blackwell, 1946), 140-152 for general overview of Parmenides’ philosophy.
 Feser, Scholastic Metaphysics, 31-32; cf. Reginald Garrigou-Lagrange, Reality: A Synthesis of Thomistic Thought (Ex Fontibus Co., 2015), 33.
 Reginal Garrigou-Lagrange, Reality: A Synthesis of Thomistic Thought (Ex Fontibus Co., 2015), 33.
 George P. Klubertanz, Introduction to the Philosophy of Being, 2nd ed. (Eugene, OR: Wipf & Stock, 2005), 79.
 Ibid., 85.
 Ibid., 125.
 For a further explication of this distinction see Klubertanz, Introduction to the Philosophy of Being, 84-87; cf. Wuellner, Summary of Scholastic Principles, 120.
 See James J. Cassidy, “The Essential Van Til – Aquinas and Barth: Their Common Core,” Reformed Forum, March 26, 2018. ; cf. Michael Hall, “The Essential Aquinas – God and the Creature: Their Absolute Distinction (A Response to James J. Cassidy of ‘Reformed Forum’),” The Reformed Philosopher, August12, 2018. .
 Wuellner, Summary of Scholastic Principles, 54-55.
 Feser, Scholastic Metaphysics, 38.
 Klubertanz, Introduction to the Philosophy of Being, 125.