The great philosopher, Aristotle, came up with a theory for the concept of change. He said that change is movement of matter from potency to act, or potentiality to actuality. The potentiality of something is the potential that that thing has to be something else. Actuality is the state of having realized that potential. Matter cannot move from potentiality to actuality without some external intervention. For example, a pot of cold water has the potential to boil, and if that same pot were boiling, its potential to boil would have been actualized. A pot of cold water, however, is incapable of realizing its potential to boil on its own. There must be an external heat source to actualize the water’s potential, like fire. Therefore, fire is the actualizer that realizes water’s potential to boil.
An object moving from potentiality to actuality is part of one of two different types of series; either an accidentally ordered series or an essentially ordered series. In an accidentally ordered series, the change of one member from potentiality to actuality is independent of its preceding member. For example, when a father goes from the potential of having a son to the actuality of having a son, he makes this change independently from his father, which is his preceding member. In an essentially ordered series, the change of one member is directly caused by the change of its preceding member. To go back to the water analogy, water cannot boil without its preceding member, fire. Fire cannot exist without a preceding member, such as a match, and so on. Thomas Aquinas will take these ideas formulated by Aristotle, and use them to prove the existence of God.
Thomas Aquinas will say that for every essentially ordered series, there must be a first member that actualizes all subsequent members of its series. As an example, consider a hand pushing a stick which in turn is moving a stone. The movement of the stone is not independent of any external force, because it moves only because the stick pushes it. Likewise, the stick is being moved by the hand, not by any power of its own. This series continues, but must stop somewhere because it cannot continue forever. Without an initial actualizer, nothing can change from potency to act because even an infinite number of members cannot actualize themselves. An infinite number of sticks will never be able to move a stone. Therefore, in order for this very first member to be indeed the first member, it must not be actualized by anything, because then it would not be the first member. It must be, according to Aquinas, pure act, having all its potentials completely realized. This is the way of the unmoved mover, or the unactualized actualizer, which is God. This way of proving the existence of God is one of the quinque viae, or “the five ways”, presented in Thomas Aquinas’s book, the Summa Theologica.
From this point, having proven the existence of God, Thomas Aquinas will proceed to derive the attributes of God. We will only discuss three, for there are many. First, God is one. If there existed more than one God, there would also have to exist a way of distinguishing them from each other. In order to distinguish one thing from another, one of the things must lack an attribute that the other has, in other words, one thing must have an unactualized potential that has been actualized in the other. This is impossible, because God by definition has no unactualized potentials, because then He would not be the first actualizer. Secondly, God is all-powerful. Reverting back to another Aristotelian principle, a cause cannot give what it does not have. God must posses all of the attributes which He actualizes as the first mover. If God possesses all attributes, then He is all-powerful by definition. Finally, God is perfect and all good. If God possesses all attributes, then He has to posses the attribute of goodness. An incorrect conclusion which may be drawn from the second attribute discussed here is that if God possesses all attributes, then he must possesses stupidity, deafness, injustice, etc. The answer to this is that these negative qualities are not attributes, but privations. Stupidity is a privation of intelligence, deafness is a privation of hearing, and injustice is a privation of justice.
Thomas Aquinas was an extraordinary thinker who proved the existence of God using only logic and reason, with the assistance of the ideas of an equally great mind, Aristotle. Aquinas’s proofs and writings have shaped Christian thought by doing the seemingly impossible: reconciling faith and reason.