By Daniel Roberts,
Earlier this week, Southern Evangelical Seminary hosted its first Open House of the year! On-campus students received a t-shirt citing one of Thomas Aquinas’ famous distinctions, the distinction between act and potency. As promised to the students in attendance, this article aims to provide a summary of the meanings of these terms and their relevance in the discipline of natural theology.
“Potency and Act divide being in such a way that whatever is, is either pure act, or of necessity it is composed of potency and act as primary and intrinsic principles.” 
A fundamental idea within the natural theology and metaphysics of Thomas Aquinas is the idea of motion or as we would understand it today, change.  What is it about the existence of things that allow them to change, and yet stay the same? Why is it that a tree has, within its ways of existing, the ability to go through so much change with the seasons and yet still remain the same tree? Is there some relationship that we can infer from our experience of change in things that could deductively lead to the existence of a being that does not change? (For more in-depth discussion about the relationship between change, reality, and our knowledge of reality, see Dr. Doug Potter’s Article, Reality: It’s Not Just a Good Idea).
The quote above is a concise description of the relationship we commonly call “change.” Thomas Aquinas, following in the tradition of Aristotelian metaphysics, argues that things that begin to exist (i.e., contingent things), are divided into two aspects of their existence. These two principles are called act and potency. A clear example of this relationship can be found in our experience of an oak tree. Its act, as an existing thing, entails everything that it is as it exists in the immediate: its branches, color, density, as well as its potential to be firewood, or a cabinet, or a wooden table, or a child’s play thing, or a dog’s play thing. All the aspects that the tree could be in the future, but is not immediately is designated as a thing’s potential, or potency.
Thus, Thomas Aquinas describes change as a thing moving from potency to act. Or said another way, contingent things change from what is possible or potential to what exists or is actual. Thomas Aquinas described the relationship this way,
“For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.” 
If this relationship is true, then one can deduce the existence of a being that is purely actual, a being that without any potential or potency. Such a being would be all-powerful, because he could not have the potential to gain any power. This being would be all-knowing, since it could not gain any more knowledge. Other attributes can be argued from this position, but the main contention is that a being that is pure actuality must exist necessarily by virtue of its essence. This is because a potency and an act cannot exist in the same being in the same sense. That is, a log cannot both be actually on fire and have the potential to be on fire. Thomas Aquinas continues his thought:
“Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold.” 
In summary, the terms ‘act’ and ‘potency’ within Aristotelian-Thomistic thought are used to describe the fundamental metaphysical relationship known as change. If a philosopher grants that this relationship corresponds to the way things are in the world, then he is quickly headed towards the world-view of theism.
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1. Listing of the 24 Thomistic Theses: http://www.catholicapologetics.info/catholicteaching/philosophy/thomast.htm#_ftn1
2. It is a common misconception that what Thomas means by this term is the idea of Newtonian motion. However, the way in which Thomas is using this word is related to the relationship of change, rather than that of loco-motion.
3. Aquinas, Thomas (2010-06-19). Summa Theologica (Complete & Unabridged) (p. 10). Coyote Canyon Press. Kindle Edition.
5. Photo Credit: Fire and Ice