In his post entitled The Essential Van Til — Aquinas and Barth: Their Common Core, Cassidy seeks to flesh out Van Til’s connection of Aquinas and Barth— “Yet the Aristotelianism of Rome, with its idea of potentiality, offers, we are bound to think, a point of contact with the underlying philosophy of Dialecticism.”  Cassidy, ultimately, pins Van Til’s statement on their “bringing God and man under a common ontological reality (being for Thomas, time for Barth).” While I cannot speak to his presentation of Barth’s position, I do disagree with his presentation of Aquinas. It is my contention that Cassidy fails to adequately grasp that which necessarily follows from and is formally contained in Aquinas’ formulation of the act-potency distinction, specifically with regard to the analogia entis and the Creator-creature distinction.
Cassidy takes Aquinas’ act-potency distinction to entail a scale of being (the concept of which he takes to be synonymous with that of the analogia entis). This is a conclusion he makes based upon his understanding of the act-potency distinction. He says, “Potency is understood opposite of actuality. And every thing has potency, which means it has potential toward actualization. Only God is pure actuality, having no potency in himself. Everything else is on its way toward actualization.”  From this understanding of the act-potency distinction, Cassidy presents his understanding of the analogia entis as necessitating an analogical relation between God and man. Both are subsumed under a common category: being (by implication, universal being without reference to their respective quiddity). Cassidy concludes that, according to the Thomistic formulation, God differs only quantitatively from man without any qualitative difference. For him, this is the precise point of commonality between Barth and Aquinas, since they both “[place] God and man in a relative relationship rather than an absolute one… [that is, they bring] God and man…under a common ontological reality (being for Thomas, time for Barth).”
It is this presentation of act and potency and the conclusions he draws from it that I take chief issue with.
First, he defines potency as “opposite of actuality” and “potential toward actualization.” This shows either a shallow reading or a misunderstanding of Thomas and Thomists on two accounts. 1.) Potency is not opposite from actuality. Rather, potency is a middle point between nonbeing and one kind of being (e.g., actuality). In other words, the distinction between potency and act is that of one kind of being from another. Their collective opposite is nonbeing. Act and potency are both of positive ontological value. For, that which is in act with regard to a particular perfection is in a state of being wherein that perfection is real or actual within that being. Correspondingly, that which is in potency with regard to a particular perfection is not in act towards that perfection, but nevertheless, it is in a state of being wherein there is a capacity to receive that perfection or act. This is simply to say that actuality in a being is a real state of being and that potentiality is a real capacity for the actualization of that perfection. Nonbeing is their opposite insofar as nonbeing is nothing. There is no positive ontological value for nonbeing.
This is particularly evident from the historical roots of the distinction, specifically that of Parmenides and Heraclitus on the issue of being (or ‘permanence’) versus becoming (or ‘change’) and Aristotle’s response to them.  Parmenides argued from the principle of contradiction that all becoming (i.e., change) is impossible, since it would require being to arise from nonbeing. This would be impossible since ex nihilo nihil fit. Thus, he ends in a static monism. Heraclitus stood opposite of Parmenides by asserting that all that is, is becoming.  It is in response to these two positions that Aristotle presents his distinction between act and potency. He solves the problem of change by asserting that change is not being arising from nonbeing (as Parmenides wrongly assumed) but rather a process wherein one kind of being (i.e., act) arises from another kind of being (i.e., potency). From these two lines of inquiry arise clearer definitions: actuality is a state of being wherein a perfection is realized, or actual in a being; potentiality is a state of being wherein there is a capacity to receive a perfection, or actuality.
2.) In his presentation of potency as “potential towards actualization” and of beings which have potency as “on [their] way toward actualization,” Cassidy seems to infer that the relation of potency to act is indiscriminate with reference to the thing’s nature. This is particularly clear when he concludes that act and potency entail a scale of being wherein there is only quantitative difference between beings on the scale of universal being—as though the only difference between two beings on the scale is more or less actuality. However, this is not a correct understanding of potency’s relation to act in a particular thing, or a correct understanding of Thomistic metaphysics more generally. All potency is grounded in the subject’s act—that is, in the nature of the thing itself. Or, at the very least, it is located in the things esse (i.e., act of existence: that by which it is). As Feser notes, “real potencies…are grounded in the natures of real things.”  There is a distinction between what Feser calls “pure or logical possibility” and “real potency.” The former is merely possible insofar as the idea of the thing (e.g., a unicorn) does not contain any contradictions in itself. Yet, it does not correspond to any extra-mental reality. The latter is properly called potency since it refers to an extra-mental reality (e.g., something in act insofar as it exists and is an essence) in which there is genuine possibilities that are not actualized. However, these potencies are not at variance with the thing’s nature. Otherwise, they would not be genuine potencies. Potency is not indiscriminate to the subject’s nature. Rather, it is constrained by the thing’s nature. A thing’s nature (that is, its act of existence and its essence) is that which determine what it is in potency. Or, in God’s case, that there is not and cannot be any potency.
Second, from this faulty understanding of act-potency and its relation to individual subjects, Cassidy argues that Thomists believe in a scale of being wherein there is only a quantitative difference between God and man. “While there is much dissimilarity between God and man—God is fully actualized, we are not—there is also a commonality as well: God and man are both beings…. And while God and man differ quantitatively in their being they are not qualitatively different.” As though the dissimilarity between pure act and compositions of act and potency is merely the quantity of actuality in each! It does not follow from the first premise (that there is a distinction between act and potency as states of being) and from the second (that there is also a distinction between God as pure act and man as composition of act and potency) that God and man differ quantitatively. The difference between pure act and composition of act and potency is not merely more actuality. That is a gross oversimplification which results in a fundamental misunderstanding of Thomistic metaphysics. On the contrary, the difference between pure act and composition of act and potency is one of simplicity and composition, of immutability and mutability, of infinitude and finitude, and so forth. It is a difference which refers to their very quidditas—that is, the very whatness of their being. This difference, which really exists between that which is pure act and that which is composed of act and potency, is not merely accidental to their respective quiddity. To be pure act is, ultimately, to have no distinction in one’s being between their esse (i.e., act of existence—that by which they are) and their essentia (i.e., essence—that by which they are what they are). Thus, when we say that God is actus purus, we mean that in God that by which he is and that by which he is what he is do not differ: that is to say, God is (esse) by virtue of that by which he is what he is (essentia), and God is what he is by virtue of that he is. This necessitates a categorical difference between God and man, one which refers to their very quiddity. For in man, there is a true and real distinction between his act of existence and his essence. Quite simply, man does not exist by virtue of his essence. Cassidy does not give the due weight to this difference between God and men that Aquinas argues for. It is in this that Cassidy goes wrong. Only of accidental properties can there be a quantitative distinction, since 1.) only accidental properties can be possessed according to measurable quantity (i.e., one man may be stronger than another. However, this does not mean that they differ in their essence. Only their possession of the accidental property of strength differs in quantity) and 2.) a distinction which refers to the essence of two differing things (such as the one between pure act and composition of act and potency) is, of necessity, a qualitative distinction.
While it is true that the medium for Cassidy’s writing is such that “a fuller scholarly treatment of this subject is beyond [his] purview,” that does not excuse his oversimplification and misrepresentation of Aquinas’ understanding of the act-potency distinction and its implications for the Creator-creature distinction. The first three of the twenty-four Thomistic theses are clear upon this issue: “1 Potency and act are a complete division of being. Hence, whatever is must be either pure act or a unit composed of potency and act as its primary and intrinsic principles. 2 Because act is perfection, it is limited only by potency which is a capacity for perfection. Hence, a pure act in any order of being exists only as unlimited and unique; but wherever it (act) is finite and multiplied, there it unites in true composition with potency. 3 Therefore the one God exists in the absolute order of existence itself, as one and most simple. All other things which participate in being itself have a limited nature composed of two really distinct principles, namely essence and existence.” 
 Cornelius Van Til, The New Modernism: An Appraisal of the Theology of Barth and Brunner (Philadelphia, PA: Presbyterian and Reformed Publishing Company, 1947), 8.
 See Garrigou-Lagrange, Reality: A Thomistic Synthesis, trans. Cummins, 32-33; Feser, Scholastic Metaphysics, 31-35.
 Here, there is a dynamic identification of being and nonbeing.
 Feser, Scholastic Metaphysics, 38-39.
 Wuellner, Summary of Scholastic Principles, 120.