From the time of Aristotle various philosophers have used the terms "Actuality" and "Potentiality". In Thomistic philosophy it is vitally important to understand what these terms mean because Thomas Aquinas relied heavily on them in the development of his systematic theology. Below is a quick introduction to what these terms mean.
In our observations we find that different beings have different capabilities. For example, rocks do not contemplate the universe: people do. Likewise, birds have the natural ability to fly, humans do not. In fact, the reason we say that it is impossible for people to fly on their own is because we know what man is naturally able to do. It would appear then that what we are places some limitations on what we can do.
The limits of what a being can do or become may also be referred to as a being’s nature. One kind of nature allows a creature to fly. Another kind of nature allows a creature to contemplate. It may also be said that one nature has the potency of flight while the other nature has the potency of contemplation. Potency in this sense is called operative potency because it is the potential of some nature to act or operate in a certain way.
Another kind of potency deals with the potential for a being to be this kind of thing or that kind of thing. This potency is called passive potency. Passive potency is that potency which allows marble to be a sculpture. A ball on a pool table is in (passive) potency to being moved by a pool player. This potency then, receives an action rather than being the cause of the acting.
If potency receives action, from what does it receive that action? Potency is always in relation to some other principle as the thing that receives that principle. The principle that actualizes potency is called act or actuality. Before a bird takes flight, that bird is in potency to flight but has not yet actualized that potency. When the bird begins to fly, that bird is no longer in potency to flight, it is actually flying. The potency in the bird while the bird was resting was very real, but it became actual rather than potential when the bird took flight.
We see then that when a thing is in act it is not at the same time in potency. Conversely, when a thing is in potency, it is not in act. To say that some being is both in act and in potency with regard to some particular activity, is to speak a contradiction. It would be like saying that a bird is both flying and not flying at the same time and sense. Therefore we see that act is related to potency, but the relation is a relation of opposition. A being is either in act or in potency to some given activity or state of being.
Potency is discovered through act because a potency does not exist as such; rather, it exists as the limiting principle of act. Act is that which makes something be, but potency determines the kind of being that will be. Act has primacy over potency in that it is that which brings about the perfection of existence. Potency does not bring about existence; it merely receives existence from act.
