Descartes' vs. Anselm's Ontological Arguments

1. What is an Ontological Argument?

Ontological arguments are a unique class of arguments for God’s existence. They stand apart from cosmological arguments (which argue from the existence of the universe to a first cause) and teleological arguments (which argue from the appearance of design in nature to a designer). The term "ontological" comes from the Greek ontos, meaning 'being' or 'existence'. The argument seeks to prove that God's existence is a necessary truth, derivable from analyzing the concept of God.

Let's begin with a short video that introduces the core idea of an ontological argument.

As the video explains, the claim is that "God exists" can be shown to be true by definition, much like a mathematical axiom. The first and most famous attempt to formalize this was made by St. Anselm in the 11th century.

2. Anselm's Argument: "That Than Which Nothing Greater Can Be Conceived"

St. Anselm of Canterbury (1033-1109) was a theologian and philosopher whose motto was fides quaerens intellectum—"faith seeking understanding." He wasn't trying to convince himself of God's existence; he already believed. His goal was to find a rational demonstration for a truth he held on faith. His argument, presented in his work Proslogion, is a beautiful piece of logical reasoning.

The core of Anselm's argument can be reconstructed as a reductio ad absurdum—a proof that works by showing that the opposite position leads to a contradiction.

The following video section explains this argument in detail.

To summarize the argument in a more formal, step-by-step manner:

1. We define God as "that than which nothing greater can be conceived."
2. Even an atheist (whom Anselm calls "the fool") understands this definition. Therefore, this concept exists in the understanding.
3. A thing can exist either in the understanding alone, or in both the understanding and reality.
4. It is greater to exist in both reality and the understanding than in the understanding alone.
5. Hypothesis for Reductio: Suppose God exists only in the understanding.
6. If this were true, then we could conceive of a greater being: one that has all of God's properties and exists in reality.
7. This would mean we can conceive of something greater than "that than which nothing greater can be conceived." This is a logical contradiction.
8. Therefore, the initial hypothesis (Step 5) must be false.
9. Conclusion: God must exist not only in the understanding but also in reality.

Almost immediately, Anselm's argument faced a powerful objection from a fellow monk, Gaunilo of Marmoutiers.

Gaunilo's "Perfect Island" Objection

Gaunilo argued that if Anselm's logic were sound, it could be used to prove the existence of all sorts of non-existent perfect things. This is another reductio ad absurdum.

Gaunilo's point is that we can conceive of a "perfect island," an island than which no greater can be conceived. Following Anselm's logic, this island must exist in reality, because an island that exists in reality is "greater" than one that exists only in the mind. Since it's absurd to think this proves the island's existence, the form of the argument must be flawed.

Anselm replied that the argument only works for God, because only God has necessary existence, whereas the existence of an island is contingent. The debate, however, has echoed through the centuries.

3. Descartes' Version: Existence as a Perfection

Centuries later, René Descartes revived the ontological argument, but for a very different purpose. As we've discussed, Descartes needed to prove God's existence to serve as the guarantor of his epistemology. His version, found in the Fifth Meditation, is subtly but significantly different from Anselm's.

Descartes' argument doesn't begin with a definition in the same way as Anselm's. Instead, it begins with an inspection of an idea he finds within his own mind: the idea of a supremely perfect being.

Here is a reconstruction of Descartes' argument:

1. I have an innate idea of God as a supremely perfect being (i.e., a being that possesses all perfections).
2. Existence is a perfection. (To exist is more perfect than not to exist).
3. Therefore, a supremely perfect being cannot lack the perfection of existence.
4. Conclusion: God must exist.

Descartes famously compares this to a geometric proof. He argues that he can no more conceive of a supremely perfect being that lacks existence than he can conceive of a triangle whose three angles do not sum to 180 degrees, or a mountain without a valley. For Descartes, existence is part of the very essence of God.

The following video provides a detailed walkthrough of the argument as Descartes presents it in Meditation V.

4. Comparison: Anselm vs. Descartes

While both are ontological arguments, their structure, motivation, and philosophical underpinnings are distinct.

Here's a summary of the key distinctions:

The most significant philosophical difference lies in the epistemic basis. Descartes isn't just playing with words; he's claiming to intuit a necessary truth about the content of an idea that, he argues, was implanted in him by God.

5. Kant's Devastating Objection: Existence is Not a Predicate

For centuries, philosophers debated the merits of these arguments. But in the 18th century, Immanuel Kant offered a critique that many consider fatal to all ontological arguments. He argued that the mistake made by both Anselm and Descartes was to treat "existence" as if it were a property or "predicate" like "is powerful" or "is blue."

Kant's point is that existence doesn't add anything to the concept of a thing. The concept of "one hundred real dollars" is identical to the concept of "one hundred possible dollars." Saying the dollars "exist" doesn't add a new feature to the dollars themselves; it asserts that the concept is instantiated in the world.

To put it in terms of formal logic, which you've studied, existence is typically treated as a quantifier ($\exists x$), not a predicate ($P(x)$). When we say "tigers exist," the logical form isn't "For all x, if x is a tiger, then x has the property of existing." Rather, it's "There exists at least one x such that x is a tiger" ($\exists x(Tiger(x))$).

If Kant is right, then "existence" cannot be a perfection or a component of greatness, because it's not a property at all. It cannot be part of the essence or definition of God, and therefore the entire ontological argument, in both its Anselmian and Cartesian forms, collapses. You cannot build instantiation into a concept and then claim the concept must be instantiated.

Conclusion

In this lesson, we have reconstructed two of the most elegant and controversial arguments in the history of philosophy. They represent a purely rationalist ambition to deduce profound metaphysical truths from first principles.

Key Takeaways:

• Ontological arguments are a priori proofs for God's existence based on the analysis of the concept of God.
• Anselm's version defines God as "that than which nothing greater can be conceived" and argues via reductio ad absurdum that such a being must exist in reality, as this is "greater" than existing in the mind alone.
• Descartes' version defines God as a "supremely perfect being" and argues from his innate idea of God that existence, being a perfection, must be part of God's essence.
• The arguments differ primarily in their motivation (theological vs. epistemological) and their epistemic grounding (a public definition vs. a private innate idea).
• Major objections include Gaunilo's "perfect island" parody and, most famously, Kant's argument that existence is not a predicate and thus cannot be part of a thing's definition or essence.

Preview of the Next Lesson:

Descartes, of course, was unconvinced by these objections and considered his proof successful. With the existence of a perfect, non-deceiving God secured, he finally had the keystone for his entire philosophical system. But not all rationalists who followed him drew the same conclusions. In our next lesson, we will turn to Baruch Spinoza, a thinker who took Descartes’ rationalist principles and his concept of substance to their ultimate, radical conclusion. We will explore his astonishing metaphysical system, where he argues for a single, infinite substance he calls "God or Nature" (Deus sive Natura), a system that stands in stark contrast to Descartes' dualism and traditional theism.