Unit 4: Philosophy of Religion

Lesson 18: The Ontological Argument

Lesson Objectives

After you have successfully completed this lesson, you should be able to do the following:

q       Explain accurately the ontological argument for the existence of God.

q       Explain accurately Gaunilo’s and Kant’s criticisms of the ontological argument.

q       Critically evaluate the ontological argument, assessing its strengths and weaknesses.

q       Explain and apply the key concepts introduced in the lesson.

q       Evaluate, articulate, and rationally defend your own position regarding the effectiveness and significance of the ontological argument.

How to Proceed                                                                                      

q       Read Section 4.3 in Lawhead together with the Reading Guide below.

q       Reflect upon the questions in the “In the Agora” section below and post your thoughts on the discussion board.

q       Complete the Mastery Check.

Reading Guide

The ontological argument for the existence of God presented in this chapter attempts to demonstrate that belief in God is rational in the strongest sense possible: that it is logically impossible for God not to exist; that any rational person must believe in God by the very meaning of the word “God.” If the argument is sound—and some philosophers believe that in one version or another it is—then atheists are entirely irrational, as if they were claiming that there are round squares or triangles with five sides. See the table in Lesson 16 to see where the ontological argument lies on the spectrum of responses.


What do you think the practical effect on the world and on missionary work would be if the ontological argument were universally understood and accepted as sound? Think about it as you read the assignment.

Section 4.3

The ontological argument, of all the traditional arguments for the existence of God, is the most difficult to understand. But if you will read carefully, do the thought experiments, and take time to ponder what you read, you will be able to understand it. Be careful not to dismiss the argument before you understand it. Seek first to understand; only then may you evaluate the argument.


P. 337  The first leading question is important. Does logic or reason tell us about reality? Think back to our unit on epistemology and the debate between rationalists and empiricists. Rationalists like Descartes claim that necessary and universal (or a priori) principles—like “every event has a cause” and “triangles have three sides”—tell us about the way things are in reality. Empiricists like David Hume claim that these only describe how the ideas in our head are related, but that they convey nothing about any reality independent of our ideas. Who is right? If I told you that I had a round square in my pocket, would you have to look to make certain I did not? Or could you be certain a priori that I had no such thing in my pocket? Do the a priori concepts of reason tell us about the existence of things in the real world outside of our minds? Think about it.


P. 338  Be sure to think through the “Thought Experiment” carefully. This is a key to your understanding the ontological argument. Do you think the notion of a logically necessary being makes just as much sense as the notion of a logically impossible being?

Lawhead points out that Anselm, like most of the philosophers who develop arguments for God’s existence, was already a believer. His belief was not based on this or any other argument.  Anselm quotes the early Christian theologian Augustine of Hippo, who declared, “Credo ut intelligam” (“I believe so that I may understand”). In other words, faith must precede philosophy—to understand God one must first believe in him. Implied in this principle is the expectation that faith is ultimately rational.

One hundred and fifty years after Anselm, Thomas Aquinas, in a very interesting work entitled Summa Contra Gentiles, conceived of the relationship between faith and reason in this way: Besides the many truths we can know by reason and the evidence of our senses, such as truths about the natural world, we also profess truths about God and the supernatural world. Some of these theological truths are beyond reason’s ability to demonstrate or even to understand, such as the essence of God or the immortality of the soul. These we must believe on faith alone from revealed sources. Others we can prove through reason, such as that God exists and that he is eternal. These beliefs we may arrive at both through faith and through reason. Most people must believe these things on faith, lacking the capacity, time, or disposition to demonstrate their truth through reason. However, it is always preferable, for those with the capacity, to establish first principles through reason. Although simple faith will satisfy most people, Aquinas believes that the God-given light of reason is intended to give us a firmer grasp of the truth and to rout out error. Finally, it is impossible, says Aquinas, for the truths of faith to be contrary to the truths of reason.

Anselm is attempting to make his faith rational, to tie it down (as Plato says) with a rational account. The life of faith is adequate, but even better is to have faith and then to understand the principles of that faith through reason, so far as human reason is able.

Do you agree with Anselm and Aquinas about this relationship between faith and reason? Think about it as you complete the rest of this unit.


P. 339  After you have read the excerpt from Anselm’s Proslogium, make certain you can answer the reading questions.

Why does Anselm claim that it is a “fool” who would deny God’s existence, besides the reference to the psalm?

Lawhead’s formalization of the argument does not represent Anselm’s actual argument very well. I therefore include a version that more closely represents Anselm’s argument. Examine each of the premises in Anselm’s argument. Make certain that you understand each premise. When you grasp the argument, you may go on to evaluate it. Do you find any of the premises unacceptable? If so, why?


Anselm’s Ontological Argument

1.      Even the fool (atheist) understands the notion of that being than whom none greater can be conceived (or greatest conceivable being).

2.      Whatever is understood exists in the understanding.

3.      Therefore the greatest conceivable being exists in the understanding.

4.      Suppose that the greatest conceivable being exists in the understanding alone.

5.      Such a being can also be conceived to exist in reality.

6.      A being that exists in reality is greater than a being that exists only in the understanding.

7.      Therefore, a greater being than the greatest conceivable being can be conceived.

8.      But this is absurd.

9.      Therefore the greatest conceivable being must necessarily exist both in the understanding and in reality.


Notice that Anselm’s is a reductio ad absurdum argument.


P. 340  The fact that Gaunilo, a Christian monk and believer, can object to Anselm’s argument, should remind us that philosophy is concerned not simply with our beliefs, but with the reasons for those beliefs. Many philosophers (such as Gaunilo) agree with the conclusion of the ontological argument (“God exists”), but disagree with this specific argument for that conclusion.

Anselm himself wrote a reply to Gaunilo, in which he states that the ontological argument will work only for the greatest conceivable being, not for things like the greatest conceivable island, or the greatest conceivable pizza, and so forth. I think Anselm has a very good point. What do you think?

Kant’s is perhaps the strongest argument against this version of the ontological argument. What is Kant’s point? Be sure to do the “Thought Experiments” on this page.

Some recent versions of the ontological argument appear to overcome Kant’s objection. We will not discuss them in an introductory course. However, the ontological argument continues to be tremendously interesting to philosophers, and some are convinced that, in one form or another, the argument is a compelling ground for believing in God.


P. 341  Read through the “Lens” section. What do you think about these implications of the ontological argument?

Carefully consider each of the positive and negative evaluations of the ontological argument. Which is the strongest positive evaluation? Which is the strongest negative evaluation? How would Anselm reply to this objection?

In the Agora

Is the ontological argument sound? You might attempt to reply as clearly and effectively as possible to Gaunilo’s or Kant’s objection. Or try to respond to one of the positive or negative evaluations at the end of the section.

Mastery Check

Look again at the objectives for this lesson. Are you able to do all of them? If not, review until you can. Then take the online quiz entitled “Quiz for Lesson 18.” You’ll find it under “Assignments” / “Quizzes” on the Bb site. Be sure to survey the key concepts for this lesson at the end of the chapter.



Continue to Lesson 19