The Ontological Argument's Weakness
It is surprise that the book entitled, Classical Apologetics,1 uses Anselm's 2 Ontological argument in its defense of natural theology. This is surprising because most classical apologists feel that this is fundamentally an unsound argument. First, a priori arguments fail because of the insurmountable difficulty of going from the realm of ideas to the realm of existence. By contrast, theistic arguments that are a posteriori (reasoning from effect to cause) begin in existence and end in existence. Hence, in the latter approach, the existence of God is proved when the arguments are sound. Second, "God" is a term used in the ontological argument which remains unknown or devoid of meaning without using the cosmological argument. In natural theology "God" is defined through a knowledge of His effects, i.e., His creation. Hence, the ontological argument begs the question, because, when a Christian uses this argument, his own view of "God" is hidden within its premises.
Thomas Aquinas notes this weakness in his Summa Contra Gentiles.
What is more, granted that everyone should understand by the name God something than which a greater cannot be thought, it will still not be necessary that there exist in reality something than which a greater cannot be thought. For a thing and the definition of a name are posited in the same way. 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. 3
Professor Holloway discusses this fallacy too.
Obviously, when a person hears the word "God" and forms some idea or meaning for this word in his mind, it by no means follows that therefore God must exist in reality. To begin with, even among those who would grant that God exists, not all would say that their concept of him is that of being greater than which we cannot imagine or conceive. The pagans, for example, who held the existence of many gods, did not conceive of God in this way. And some who have held one God thought that he was the world, or nature, and so forth. So that it is simply not true that when a person hears or thinks of the word "God," he thinks of something greater than which cannot be conceived.4
The cosmological argument begins with the reality of existent things, i.e. the creation. It argues from effects (contingent beings) to the ultimate cause (a necessary eternal being) of contingent being. This manner of approach to God's existence and His attributes is supported in scripture. The scriptural starting point is "the things that are made." This is what all of mankind sees and provides a beginning point to argue a posteriori to God's existence and His nature. To begin in the realm of Platonic Ideas fails, because ideas alone don't cause existence. For example, intellectually knowing accounting principles (realm of ideas) does not mean there exist coins (realm of existence) in an accountant's pocket.
For the invisible things of him since the creation of the world are clearly seen, being perceived through the things that are made, (even) his everlasting power and divinity; that they may be without excuse: Rom 1:20 (ASV)
The heavens declare the glory of God; and the firmament sheweth his handywork. Day unto day uttereth speech, and night unto night sheweth knowledge. There is no speech nor language, where their voice is not heard. Ps 19:1-3 (KJV)
1 Sproul, R.C., J. Gerstner, A. Lindsley, Classical Apologetics: A Rational Defense of Christian Faith and a Critique of Presuppositional Apologetics, Academie Books, Grand Rapids, MI, 1984, 364.
2 St. Anselm, Proslogion, with A Reply on Behalf of the Fool by Gaunilo and The Author's Reply to Gaunilo, Translated by M.J. Charlesworth, University of Notre Dame Press, (Reprint of Oxford University Press, 1965), Notre Dame, London, 1979, pp. 196.
3 St. Thomas Aquinas, Summa Contra Gentiles, Book One, God, Chapter 11. Translated by Anton C. Pegis, In: On the Truth of the Catholic Faith, Image Books, Garden City, NY, 1955, p. 82.
4 M. R. Holloway, An Introduction to Natural Theology, Appleton-Century-Crofts, Inc., New York, NY, 1957, p. 47