Aquinas' Five Ways

Introduction

Welcome to your seventh lesson on Medieval Western Philosophy. In our last session, we grappled with Anselm's ontological argument, a purely a priori attempt to prove God's existence from the definition of God as "that than which nothing greater can be conceived." We saw how this rationalist proof was challenged by Gaunilo's "Lost Island" parody, which questioned the very logical form of the argument.

Today, we turn to a towering figure of the era, Thomas Aquinas (c. 1225–1274), who took a fundamentally different approach. Whereas Anselm argued from reason alone, Aquinas believed that proof for God's existence must begin with what we can observe in the world through our senses. His arguments are therefore a posteriori—based on experience and observation.

Your learning outcome for this lesson is to reconstruct Aquinas's Five Ways for proving God's existence from his seminal work, the Summa Theologiae. We will examine the structure of his arguments and break down the logic of each of the "Five Ways." This will provide a crucial contrast to Anselm's method and showcase the influence of Aristotle's thought on medieval philosophy.

1. From Observation to Inference: The Method of Aquinas

Aquinas's proofs are not abstract logical puzzles like Anselm's. Instead, they function as examples of what we now call "inference to the best explanation." Aquinas observes certain undeniable features of the world—motion, causality, existence itself—and argues that the best, and indeed only, complete explanation for these features is the existence of God.

Before we dive into the arguments themselves, it is useful to familiarize ourselves with some key Aristotelian concepts that Aquinas employs. You will recall these from our module on Aristotle, but a brief review is essential here.

Now, with this philosophical toolkit ready, let's turn to the primary source. The Five Ways are presented in a single article within Aquinas's monumental work, the Summa Theologiae.

2. Reconstructing the Five Ways

Having read the original text, we can now reconstruct each argument into a more formal, step-by-step structure. This will clarify the logical progression from premise to conclusion. The first three arguments are often grouped together as cosmological arguments, as they reason from the nature of the cosmos.

The First Way: The Argument from Motion

This argument begins with the simple observation that things are in motion or, more broadly, changing.

• Premise 1: We observe that some things in the world are in motion (i.e., changing from potentiality to actuality).
• Premise 2: Whatever is in motion is put into motion by another thing that is already in act.
• Premise 3: A thing cannot be simultaneously in act and in potentiality in the same respect; therefore, nothing can move itself.
• Premise 4: This chain of movers cannot go on to infinity, because then there would be no first mover, and consequently, no subsequent movers.
• Conclusion: Therefore, it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

This "First Mover" is an Unmoved Mover, a concept Aquinas borrows directly from Aristotle.

The Second Way: The Argument from Efficient Causation

This argument is very similar in structure to the first but focuses on the existence of causes and effects.

• Premise 1: We observe an order of efficient causes in the world.
• Premise 2: Nothing can be the efficient cause of itself, because it would have to exist prior to itself, which is impossible.
• Premise 3: This chain of efficient causes cannot go on to infinity, because in any causal chain, the first cause is the cause of the intermediate, which causes the final effect. Without a first cause, there can be no intermediate or final effects.
• Premise 4: Since we clearly observe effects, the causal chain cannot be infinite.
• Conclusion: Therefore, it is necessary to admit a first efficient cause, to which everyone gives the name of God.

The Third Way: The Argument from Possibility and Necessity

This is the most complex of the Five Ways. It relies on the distinction between contingent beings (which can either exist or not exist) and necessary beings (which cannot fail to exist).

• Premise 1: We find in nature things that are contingent—they are generated and they corrupt, so it is possible for them to be and not to be.
• Premise 2: If everything were contingent, it is possible that at some point, nothing existed.
• Premise 3: If at one time nothing existed, nothing could begin to exist, since something cannot come from nothing. Thus, nothing would exist now.
• Premise 4: But things clearly do exist now, which is a contradiction.
• Premise 5: Therefore, not all beings are contingent; there must be at least one necessary being.
• Premise 6: This necessary being either has its necessity caused by another or it does not. This chain of necessary beings causing other necessary beings cannot be infinite.
• Conclusion: Therefore, there must exist some being having of itself its own necessity, which causes the necessity in others. This all men speak of as God.

The Fourth Way: The Argument from Gradation of Being

This argument moves from observing degrees of perfection in the world to the existence of a maximum perfection.

• Premise 1: We observe that things in the world have degrees of qualities like goodness, truth, and nobility.
• Premise 2: The terms "more" and "less" are used in relation to a maximum or ultimate standard (e.g., we judge something as "hotter" by its proximity to what is "hottest").
• Premise 3: Therefore, there must be something that is the maximum of all perfections—truest, best, noblest, and hence, ultimate in being.
• Premise 4: The maximum in any category is the cause of all else in that category (e.g., fire, the maximum heat, is the cause of all hot things).
• Conclusion: Therefore, there must be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.

The Fifth Way: The Argument from Governance of the World

This final argument is a teleological argument, or an argument from design. It reasons from the apparent purposefulness in nature to the existence of a designer.

• Premise 1: We observe that even non-intelligent things in nature (e.g., plants, planets) act towards an end or goal, achieving the best result.
• Premise 2: Things that lack intelligence cannot move towards an end unless they are directed by some being with knowledge and intelligence.
• Premise 3: This is analogous to an arrow being directed to its target by an archer.
• Conclusion: Therefore, some intelligent being exists by whom all natural things are directed to their end; and this being we call God.

Conclusion

In this lesson, you have reconstructed the five a posteriori arguments for God's existence proposed by Thomas Aquinas. You have seen how he builds his case not on pure definition, but on empirical observations of motion, causality, contingency, perfection, and design in the universe. Each "Way" begins with an observation and argues, by ruling out an infinite regress, for the necessity of a terminal point—a First Mover, a First Cause, a Necessary Being, a Maximum Perfection, and an Intelligent Designer—which he identifies as God.

Key Takeaways:

• A Posteriori vs. A Priori: Aquinas's method stands in stark contrast to Anselm's. He argues from experience, not from pure reason.
• The First Way (Motion): An Unmoved Mover must exist to explain change.
• The Second Way (Causation): An Uncaused Cause must exist to explain the existence of effects.
• The Third Way (Contingency): A Necessary Being must exist to explain the existence of contingent beings.
• The Fourth Way (Gradation): A Maximum Perfection must exist to be the standard and cause for all degrees of perfection.
• The Fifth Way (Design): An Intelligent Designer must exist to explain the goal-directed behavior of non-intelligent things.

These arguments, rooted in Aristotelian metaphysics, became cornerstones of Christian philosophy and continue to be debated and refined to this day.

In our next lesson, we will explore another major debate from the late medieval period: the problem of universals. We will analyze the nominalist solution proposed by William of Ockham and his famous principle of parsimony, "Ockham's Razor," as we move towards the philosophical shifts that will pave the way for the modern era.