Although Aquinas applies the concepts of act and potency to almost every facet of existence, from natural science to the features of the intellect, and even to the characteristics of the soul and angels, the best starting point to understand the important categories of act and potency is the relationship these ideas have with Being. A large part of Aquinas’s philosophical methodology is to distinguish and apply the categories of act and potency to all of reality. (Sometimes Aquinas uses the word “powers” to mean innate potencies or abilities.) Aquinas himself tells us that all reality, or Being, can be divided between act and potency. “Potency and act divide being and every kind of being” (400). At another point Aquinas explains, “Since Being properly signifies that something is, in act, and act is properly ordered to potentiality, a thing is, in consequence, called Being absolutely according as it is primarily distinguished from that which is only in potency; and this is each thing’s substantial Being” (23). Furthermore, Aquinas expands upon the nature of Being, “Being is the actuality of every form or nature; for goodness or humanity are spoken of as actual only because they are spoken of as being. Therefore, being must be compared to essence, if the latter is distinct from the former, as act to potency” (17). Something is “in act” or in the state of actuality when it is existing in physical reality or knowable through the intellect. Echoing Aristotle, Aquinas tells us “everything is knowable only in so far as it is in act” (24). The concept of Being which is the most general concept we have of reality, can nevertheless be delineated between act and potency.

Being, however, has two senses — one sense refers to physical objects of reality and, in the second, intellectual objects of the mind such as statements, ideas or concepts such as goodness or humanity, and mathematics. Again Aquinas explains, “’To be’ can mean either of two things. It may mean the act of being, or it may mean the composition of a proposition effected by the mind in joining a predicate to a subject” (17). In sum, all of reality exists in one way or another in act — either in acts of perception or acts of the intellect. It is important to keep in mind, also, that Aquinas does not hold to a separate realm of ideas as Plato does, or the idea that the mind itself determines reality. As will be explained later, Aquinas holds that form and matter are always united together. In his epistemology, however, Aquinas holds that the natural physical things of this world (things existing “in act”) have their own intractable reality and they are known by way of the five senses. It is through abstraction that one realizes essences and universals that exist in the intellect. For example, “redness” is realized by viewing several red objects. Aquinas, following Aristotle, is grounded in nature and believes that the physical objects of this world have their own inherent and intrinsic characteristics and that these characteristics can be known through abstraction. In other words, reality is the determinate of order. According to Aquinas, “Our intellect cannot know the singular in material things directly and primarily. The reason of this is that the principle of singularity in material things is individual matter, while our intellect… understands by abstracting the intelligible species from such matter. Now what is abstracted from universal matter is the universal” (461). Aquinas indicates that it is possible for the intellect to know the particular through reflecting on the universal. All Being, nonetheless, exists in the universals and particulars that make up the being and becoming of this world.

For Aquinas, Being includes act and potency, but how does change actually take place in the realm of becoming? More specifically, what is the cause of change? With the exception of universals, virtually all things of the physical world are subject to change or movement. As noted earlier, both living and non-living things experience change. Change is the actualization of a thing’s potential. Aquinas provides this definition of change, “A thing requires to be moved by something in so far as it is in potency to several things; for that which is in potency must be reduced to act by something actual, and to do this is to move” (658). Change, then, is potency reduced to act. However, an outside and additional element is necessary to bring about change. Potency is needed for change, but it is only a necessary condition, not a sufficient condition to bring about change. Aquinas explains, “Now it is impossible for a thing’s being to be caused by its essential principles, for nothing can be the sufficient cause of its own being, if its being is caused” (17). In other words, for an acorn to change into a beautiful oak tree, more than potential is needed. In order for an oak tree to develop from an acorn, the acorn must be nurtured by sunshine, rain, nutrients in the soil, and time. Something else, always itself in “act,” is needed to actualize a potential. “For whatever is in potency can be reduced to act only by some being in act” (14). Potency itself cannot bring about act — just as non-being cannot bring about being.

That which reduces a potential to act is called a cause and a quick overview of Aristotle’s famous four causes is, therefore, necessary. Aquinas is fully committed to the Aristotelian four causes. The first cause is the material cause — that out of which something is actualized. The second cause is the efficient cause — that by which something is actualized. The third cause is the formal cause — that into which something is actualized. The fourth cause is the final cause — that for the sake of which something is actualized. A biological example will help explain how these four causes work in the physical world. To understand the basics of the human heart, it is important to know what it is made of — its material cause. In this case, the material cause of a heart is muscle. The efficient cause rests in the DNA that structures and orders cells in such a way as to create a muscular heart and not a kidney or spleen. The formal cause is that which the heart muscle is fashioned into — ventricles, arteries, aorta, etc. The final cause serves the purpose of pumping blood. For both Aristotle and Aquinas, final causes are significant and found whenever cause and effect relationships are seen. The totality of the three previous causes all serve a goal or ultimate end, culminating in the final cause. Focusing on act and potency, the efficient cause is that which actualizes a potency. That is why Aquinas says, “Now everything which is in any way changed is in some way in potency” (38). In other words, there is always an external component to a change which reduces potency to act, and that external component is the efficient cause. The builder of a ship is the efficient cause of the ship, and the DNA in the acorn is the efficient cause of the oak tree.

All things in act, therefore, are the subject or cause of change. That which is in act is always a composition of act and potency. According to Aquinas, all things are made up of a composition. “Thus, in everything which is moved, there is some kind of composition” (39). One of the metaphysical aspects of reality is that the normal everyday things of this world are composed of act and potency. For Aquinas, another significant aspect of all of reality is the composition of form and matter. Aquinas explains, “matter is that which is in potency” (15). And this relates to change as well, according to Aquinas, “For just as matter, as such, is in potency, an agent, as such, is in act” (21). The twofold composition of form and matter is simply a restatement of the Aristotelian understanding of formal and material cause. It is also important to understand that things which change according to a final cause must also be composed of form and matter. Without form there would not be an essence to something and an inherent or final nature could not be realized. Aquinas explains how this final end is related to act and form, “upon the form follows an inclination to the end… for everything, in so far as it is in act, acts and tends towards that which is in accordance with its form” (27). For Aquinas then, the immaterial aspects of causation are always connected to act and potency, and form and matter.

In addition, that which is in act is combined with form and matter, “being is the actuality of every form or nature” (17). Just as everything is composed of act and potency, all things are composed of form and matter. “Hence, being itself is the actuality of all things, even of forms themselves” (21). As we have seen already, being is that which is in act. We can therefore understand, that which is in act is also combined of form and matter. Aquinas explains that universals and essences can be infinite, but form is bound and contracted to matter, “Just as immaterial things are in a way infinite as compared to material things, since a form is, after a fashion, contracted and bounded by matter, so that form which is independent of matter is, in a way, infinite” (620). In other words, apart from universals, essences, or abstractions, forms are always in combination with matter and that which informs, or participates in the transformation of matter when it changes, and matter is that which participates in the potential of receiving the form. “Forms which can be received in matter are individualized by matter” (16). Echoing Aristotle, Aquinas holds that in physical objects, forms are inherent or immanent in the things themselves.

As it turns out, the concept of potency is powerful and helps to provide a foundation for understanding the metaphysical structure of things that undergo change in this world. Therefore, wider implications of the act/potency distinction warrants exploration. From physical nature to human beings, everything that is in act is also in potency. Potency, therefore, precedes and supersedes actuality and provides continuity when things change because that which is in act will change according to its nature, or essence, through potency. Potency is not something that can be measured by the tools of the physicist, but its effects can be. Science, after all, is not interested in matter in and of itself, but rather in the properties, capacities, and possibilities that the matter contains by virtue of its potency. Whenever a drug manufacturer gives a list of possible side effects (such as bleeding, restlessness, bloating, blindness, etc.) it is explaining the potentialities that might occur if certain conditions are met. In information science, we know that computers only perform as they do because electrons have certain properties and not others. Finally, we do not yet know the capacity and limitations of the human mind and intellect. The capacity for the mind to grow, develop, and learn speaks to the potential of what it means to be human. Human beings are amazing and have the ability to adapt, appropriate knowledge, and develop greater and greater abstraction. Pure potentiality can never be measured empirically, but to deny its reality would be intuitively and conceptually absurd. Aquinas’ act/potency and his correlative form/matter distinctions make the most sense out of reality and provide a significant foundation for understanding the metaphysical structure of change and the world we live in.

Works Cited

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