By Emily Evans, Holy Apostles College
In the eleventh century A.D., St. Anselm of Canterbury (1033-1109) constructed the first and perhaps most well-known ontological argument. St. Anselm of Canterbury’s ontological argument attempts to prove the existence of God from the very definition of God Himself by demonstrating the absurdity of the non-existence of God. For if the mind conceives of a being of which no greater can be thought of or conceived, such a being must exist since existence, according to Anselm, is a necessary component of perfection. If such a being were non-existent, there would be a contradiction since an all-perfect being must exist by definition. This all-perfect being, according to Anselm, is God. In spite of this, the ontological argument fails because the only being who can truly understand the essence of God is God Himself, for no being except God has an infinite knowledge of the infinite—man can only understand the existence of God through His effects.
St. Anselm’s Ontological Argument
In Anselm’s ontological argument which he formulates as a speech to God, Anselm argued that because there exists in the mind a being of which no greater can be thought of or conceived, there cannot be a contradiction between what is thought of in the mind and reality, as what exists in the mind must exist in reality. Thus, the being of that which no greater can be thought of or conceived must exist. This being of that which no greater can be thought of or conceived must be God for God is the only being of which no greater can be thought of or conceived. Therefore, God must exist. Through this line of thought, Anselm reasons for God’s existence through the use of a reductio ad absurdum argument which, according to Nicholas Rescher, “seeks to establish a contention by deriving an absurdity from its denial, thus arguing that a thesis must be accepted because its rejection would be untenable.” In Anselm’s argument, if the mind can conceive of an being who is greater that which can be thought of or conceived, such a being embodies perfection since existence is perfection. With this, Anselm argued that is absurd to argue that God does not exist since God’s perfection necessitates his existence. In other words, if the mind conceives of a being of which no greater can be thought of or conceived, such a being must exist since existence, according to Anselm, is a necessary component of perfection. If such a being were non-existent, there would be a contradiction since an all-perfect being must exist by definition. According to Anselm, it is “only the fool, the insipiens, who has said in his heart, that ‘there is no God.’” For those that understand the definition of a being of which no greater can be thought of or conceived must necessarily accept the existence of God, according to Anselm.
Objection to the Argument
With this, the great philosopher St. Thomas Aquinas (1225-1274) attacked a vital premise of Anselm’s argument; in particular, Aquinas speculated whether all men understand God in a manner sufficient for the proof of the ontological argument to be sound. According to Aquinas:
Perhaps not everyone who hears this word “God” understands it to signify something than which nothing greater can be thought, seeing that some have believed God to be a body. Yet, granted that everyone understands that by this word “God” is signified something than which nothing greater can be thought, nevertheless, it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally.
Aquinas postulated that man does not possess an implicit knowledge of the essence of God or even a distinct knowledge of the existence of God—this is because man cannot understand or grasp the nature of an all-powerful Being. Rather, the knowledge that “God exists in a general and confused way is implanted in us by nature, inasmuch as God is man’s beatitude.” Therefore, to come to know God, man must rationalize God’s existence from the sensible world since man does not and cannot perfectly understand the essence of God.
This point that the essence of God is not self-evident to man is of the utmost importance. For if the essence of God is not self-evident to man, then the ontological argument faces serious difficulties. This is because the ontological argument presupposes that all men perfectly understand God. Even if all share the same concept of God, Aquinas argues that the ontological argument does not necessarily prove the extramental existence of God. Aquinas reasons in Summa Contra Gentiles that,
Now, from the fact that that which is indicated by the name God is conceived by the mind, it does not follow that God exists save only in the intellect. Hence, that than which a greater cannot be thought will likewise not have to exist save only in the intellect. From this it does not follow that there exists in reality something than which a greater cannot be thought.
In other words, this means that one simply is aware of the mental existence of the words themselves. For according to Kenneth Einar Himma, “We have no idea of what this sequence of words [being of which no greater can be thought of or conceived] really means. On this view, God is unlike any other reality known to us; while we can easily understand concepts of finite things, the concept of an infinitely great being dwarfs finite human understanding.” 
Essentially, while one can assert that God is the being of which no greater can be thought of or conceived, one cannot come to the conclusion that this being exists since arriving at a logical conclusion necessitates that one understands the terms and premises of an argument. In the same way, if the proposition that God’s existence is self-evident to man, then, why would an argument for God’s existence be necessary in the first place as with this proposition there would be in essence nothing to prove.
So, because of this, the only way for the ontological argument to be logically valid is for one to fully understand and know the essence of God. This though is impossible as Aquinas and other philosophers have argued that man is unable to fully understand or know God’s essence because of the limitation in man’s reason and knowledge. So, while the proposition “God exists” is self-evident; it is only self-evident, according to McInerny, “through the knowledge supplied to us by faith, or by reason, through which we learn that, in God, essence implies existence.” Furthermore, Aquinas argued that this premise is only self-evident in itself. This is similar to the truth that a square is a two-dimensional object with four equal sides with four right angles adding up to three-hundred-sixty degrees. To know this truth, however, presupposes some knowledge concerning mathematics and geometry. Without a knowledge of squares, one will either be totally ignorant of squares, or one will simply form false conceptions concerning squares. In the same way, this applies to the proposition that God exists in the sense that one must have sufficient knowledge of God in order to conclude that God exists. From this, it follows that the only being who can truly understand the essence of God is God Himself, for no being except God has an infinite knowledge of the infinite. Because of this, according to Aquinas, man can only understand the existence of God through His effects, which is the basis for his five proofs for the existence of God.
If Anselm’s argument were true then, all of mankind should be convinced that God exists, yet this is not the case considering the number of atheists and agnostics in the world. This is because, as Aquinas points out, man does not have a direct knowledge of the essence of God. For if man did have a direct knowledge of the essence of God, then it would be simply impossible to deny the existence of God. This is because if man understood God’s essence, he would possess a supreme knowledge of God which would lead to the necessary conclusion of God’s existence. If man has a direct knowledge of God, man could not deny the existence of God and still be considered a rational animal, for the existence of God would essentially fall into the category of fundamental logical truths, the denial of which would undermine the entirety of man’s rationality, (i.e., the law of non-contradiction). The inevitable conclusion of this is that those who deny the existence of God must necessarily be irrational, this though is not the case. Moreover, if man possessed a perfect knowledge of God’s essence, this would necessitate that man is not limited in knowledge. Ultimately, then, Aquinas seems wary as the ontological argument possesses a kind of intellectual arrogance. For how can man, a finite fallen being properly conceive of an infinite perfect God if man himself is imperfect and limited in knowledge?
As it can be seen then, while the conclusion that Anselm derives from his argument, namely that God exists, is certainly true, the argument that he employs to reach this conclusion is unsound. For Anselm’s argument relies on man knowing the essence of God in order to prove the absurdity of the non-existence of God. Yet, since the essence of God is only self-evident to God Himself, the argument can in no way demonstrate the absurdity of the non-existence of God. This is because according to the article “Validity and Soundness,” “A deductive argument is sound if and only if it is both valid, and all of its premises are actually true. Otherwise, a deductive argument is unsound.” While the ontological argument is valid, the premise that the mind can understand the essence of God is false. This is because, according to McInerny, “Real, including divine being, is not established by the idea of real being. Real being is established only by real being.” Because of this, the ontological argument is ultimately logically unsound and philosophically non-demonstrable.
Anselm. Proslogian. Trans. Thomas Williams. Indianapolis, IN: Hackett Publishing Company
Copleston, Frederick. A History of Philosophy, Volume II: Medieval Philosophy. New York:
Himma, Kenneth Einar. “Anselm: Ontological Argument for God’s Existence.” In The Internet
Encyclopedia of Philosophy, ed. James Fieser and Bradley Dowden. At www.iep.utm.edu.
Thomas Aquinas. Summa theologiae. 2nd ed. Trans. Fathers of the English Dominican Province.
At New Advent, www.newadvent.org.
Rescher, Nicholas. “Reductio ad Absurdum.” In The Internet Encyclopedia of Philosophy, ed.
James Fieser and Bradley Dowden. At www.iep.utm.edu.
Thomas Aquinas. Summa Contra Gentiles: Book I God. Trans. Anton C. Pegis. New York City: Doubleday, 1955.
“Validity and Soundness.” In The Internet Encyclopedia of Philosophy, ed. James Fieser and
Bradley Dowden. At www.iep.utm.edu.
 Anselm, Proslogion, trans. Thomas Williams (Indianapolis: Hackett Publishing Company, Inc., 2001), 16-17.
 Himma, “Anselm: Ontological Argument for God’s Existence.”
 Frederick Copleston, A History of Philosophy, Volume II: Medieval Philosophy (New York: Doubleday, 1950), 162.
 Copleston, A History of Philosophy, 162.
 ST, I, q. 2, a. 1, ad 1.
 Thomas Aquinas, Summa Contra Gentiles: Book I God, trans. Anton C. Pegis (New York City: Doubleday, 1955), 82.
 Himma, “Anselm: Ontological Argument for God’s Existence.”
 McInerny, Natural Theology, 65.
 ST, I, q. 2, a. 1.
 McInerny, Natural Theology, 66.