Although Aristotle and Aquinas define the term “metaphysics” in various ways, one common rendering both use is “first principles.” For these thinkers, metaphysics is the study of the first principles of reality and how one views the world. One of the guiding first principles for Aquinas, especially in his metaphysics or understanding of reality, is his distinction between “act” and “potency.” Act and potency are not unique to Aquinas; these categories are first developed by Aristotle who uses these categories to describe how causation and change work in the physical world. Nonetheless, Aquinas, following Aristotle, applies the categories of act and potency to virtually every aspect of reality. In addition to act and potency, Aquinas also uses the categories of form and matter to make sense out of reality as Being. With the categories of act and potency and form and matter, Aquinas describes the immaterial structure and nature of all of reality and explains how potency accounts for change. He uses the categories of act and potency to such a degree, and so broadly, that they are among the most important principles for understanding Aquinas’s metaphysics and have broad implications to how one understands nature, science, and human potential.

For Aquinas, and many philosophers in the Aristotelian-Thomist tradition, the proper object of study for metaphysicians is the concept of Being¹. In the simplest terms, Being is the study of all that exists, including concrete physical phenomena such as humans, tables, quadrupeds, things that can be discerned from the senses, and abstract mental concepts such as mathematics, goodness, and other ideas. Being, therefore, is the study of the nature and framework of all reality. It is important to note that Being is not a particular genus or species, but rather that in which all genera and species participate. That which exists and all that which stands out of non-being, participates in Being, but Being itself is not a genus or species. “Being itself is considered as formal, and as something received, and not as that to which Being belongs” (21). As Aquinas reminds us in the most general terms, “Being is common to all” (22). It is important to note that for Aquinas, the physical and metaphysical are not separated. The physical things that exist in act or being also participate in the metaphysical notion of potency. Through act and potency, Aquinas provides an important conceptual scheme for things which participate in being and also undergo change.

Much of philosophical history is an attempt to understand the mystery of change. Although Aquinas uses Aristotle’s categories of act and potency to reveal important insights about reality and to explain how change is possible, the discussion begins with Plato. Plato, following Parmenides, insisted that true Being implies permanence. Parmenides teaches that Being and Non-Being are the only genuine realities of existence, and change, therefore, is impossible because Non-Being cannot be the cause of Being. Change, although obvious to the senses, is an illusion. Plato, influenced by Parmenides, holds that the changing and mutable things of this world are not real Being or existence in the highest sense — only the Forms existing in a transcendent realm, participate in true Being. Aristotle, however, being firmly grounded in nature and the physical world, knows that change and motion are real and must be accounted for. In Aristotelian and Thomist terms, motion means change in the broad sense and not just movement from one place to another. Drawing on Aristotle, Aquinas explains, “For because ‘motion from place to place is naturally the first of movements,’ as is proved in the Physics, we use terms belonging to local motion in speaking of alteration and movement of all kinds” (351). For Aristotle and Aquinas, physical things, existing in reality actually change. Change is part of Becoming. Influenced by Aristotle, Aquinas knows that things change according to their nature and he uses the concept of potency to explain how change is possible. Things do not change at random or whimsically, but according to their nature. For example, an acorn has the natural potential or capacity (if nothing hinders it) to become a mighty oak tree. In the case of natural or biological generation (a type of change) Aquinas states, “It ought, then, rather to be said that in the natural generation of all animals that are generated from seed, the active principle lies in the formative power of the seed…” (367). Just as biological matter has inherent tendencies towards life, Aquinas also explains that human beings have intrinsic potencies. Relating human potential to the “intellectual soul” (395) as that property humans have to comprehend universals and to use their reason to understand and create things, Aquinas explains, “Aristotle does not say that the soul is the act of the body only, but ‘the act of a physical organic body which has life potentially’; and that this potency does not reject the soul’ … In like manner, the soul is said to be ‘the act of the body,’ etc., because by the soul it is a body and is organic, and has life potentially” (394). In other words, a student has the potential to become a doctor if she studies and applies her intellectual abilities. Potency, then, contains the possibilities that something can change into according to its nature.

1 Many philosophers in the Aristotelian and Thomist traditions prefer the term “ontology” in describing their approach to the study of Being.

Works Cited

Aquinas, Thomas. The Summa Theologica, Volume 1. Great Books of the Western World. Vol. 17. Encyclopedia Britannica, Inc., 1999