Rationalism is the belief man looks to himself to gain knowledge. Rationalists hold man’s mind to be perfectly suitable alone to acquire knowledge about the world in which we live. Thinking is the key. Descartes’ “I think therefore I am.” is their well beloved adage. Rationalism holds, contrary to Empiricism, not all knowledge is attained through sensory experience, but man is born with certain innate ideas. The laws of logic are thought to be some of these; they are not learned, but rather, every man is equipped with these ideas apart from experience regardless of whether they know it.
Rationalism’s contribution to Christian apologetics is important. The ontological argument, perhaps the best, but certainly the most well known rationalistic argument, was formulated by Anselm of Canterbury (1033-1109), as an attempt to find common ground with the unbeliever. The common ground? Reason. Ontology is the study of being. The ontological argument argues from the being of God (His attributes and greatness, ect.) to prove the existence of God. The argument is in the form of a prayer, The premises of which are as follows:
1. God is something of which nothing greater can be thought.
2. God may exist in the understanding.
3. To exist in reality and in the understanding is greater than to exist in the understanding alone.
4. Therefore, God exists in reality.
Philosophers, both secular and religious, struggled with the argument for centuries, attempting to find problems with the form of the argument. Yet not until Immanuel Kant (1724-1804) do we find a satisfactory answer to the argument. This shows the pervasiveness of the argument at the time. Whether or not the argument is valid will not be examined here. It is a complicated argument difficult to understand and even more difficult to explain. Let us instead grant the validity of the argument, and ask the question: What knowledge does it justify? And again: Is it Christian?
Dr. Clark makes this observation:
“… even if the ontological argument should be valid, no one has ever succeeded in deducing the precise number of planets, or the actual species of japonica, from the existence of God by logic alone. And if astronomy and botany must progress apart from rationalism, it is inconsistent to demand that religion should be so confined.”
Which is to say, rationalism and the ontological argument are insufficient to supply man with any real knowledge. Supposing the argument valid, we are only left with knowledge of a god. But this hardly accounts for a structure of religion generally or of Christianity specifically. How do we know if God is personal? How do we know He is Jehovah? How do we know what He ethically requires or if He requires anything at all? Certainly Anselm believed in the Trinity, but how could he deduce the Trinity from an abstract concept of God? What about the atonement? Anselm made an attempt to deduce these things in his work Cur Deus homo, but “what necessity was there of God’s becoming man when he could have saved us through some less painful method.” Anselm couldn’t avoid the necessity for a written revelation for these doctrines, though he denied it.
Because epistemology is basic to all other branches of philosophy (and learning in general), knowledge of any one of these other branches (science, ethics, politics) is only as strong as the epistemology that lies behind them. If the foundational principal is flawed, subsequent deductions will also be flawed. If Rationalism or Empiricism cannot tell us where we get knowledge how can they answer the ethical, political, and sociological questions of our day? Clearly, they cannot.
 The full argument is found in Anselm’s Proslogium, Chapter 2
 Clark, Religion Reason and Revelation, 53
 Clark, Thales to Dewey, 254