The Argument from Efficient Causes as Proof of God

Thomas Aquinas’s Five Ways present a series of profound arguments for God’s existence, each addressing a different aspect of reality. The Second Way, known as the Argument from Efficient Causes, examines the sequence of causation in the universe and concludes with the necessity of a First Cause. Located in the Summa Theologica (I, Q. 2, Art. 3), this argument transcends physical explanations to explore the deeper metaphysical basis of existence, ultimately pointing to God. As contemporary philosopher Edward Feser notes, “Aquinas’s Second Way is not just a relic of medieval thought; it is a timeless challenge to atheism and materialism.”¹

The Second Way Explained: Efficient Causes and the First Cause

Aquinas begins with the observable fact that things in the world are caused by other things. However, nothing can cause itself, as this would require it to exist prior to its own existence—an impossibility. In Aquinas’s words:

“In efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause.”²

If there were no first cause, there would be no intermediate causes or effects. Thus, Aquinas argues for the necessity of a First Cause, itself uncaused, which begins the chain of causation and sustains all things. He concludes, “This everyone understands to be God.”³

This argument hinges on the rejection of an infinite regress of causes, which Aquinas sees as metaphysically untenable. As philosopher David Bentley Hart explains:

“An infinite regress of causes explains nothing. It merely pushes the question back indefinitely without ever arriving at an answer for why anything exists at all.”⁴

By grounding all causation in the First Cause, Aquinas demonstrates that the universe is contingent—dependent on something else for its existence—and that something must exist necessarily and independently.

Philosophical Foundations: Causality and Contingency

The Second Way is rooted in Aristotelian metaphysics, particularly the principle of causality. Aristotle’s concept of efficient causes refers to the agent or force that brings something into being. For instance, a sculptor (efficient cause) creates a statue (effect). Aquinas expands this principle to argue that no effect can exist without a preceding cause. However, this chain cannot stretch infinitely into the past, as it would fail to explain the current existence of anything.

As William Lane Craig elaborates:

“The Second Way demonstrates that causation cannot loop back infinitely. It demands a terminus—a ground of causality that is itself uncaused and necessary.”⁵

Aquinas’s argument does not merely address physical causation but the deeper metaphysical question of why anything exists. This distinction is crucial, as it transcends scientific explanations of the universe to address its ultimate origin. Thomist philosopher Étienne Gilson remarks:

“The genius of Aquinas lies in his ability to see that causality must be grounded in a being that is not caused, for only such a being can explain the existence of all others.”⁶

Theological Implications: God as the First Cause

The Second Way not only identifies the necessity of a First Cause but attributes divine characteristics to it. This First Cause must be unchanging, eternal, and immaterial, as it exists outside the chain of dependent causes. Moreover, it must be self-sufficient, as nothing external can account for its existence. These attributes align with the God of classical theism, who is described in Scripture as the Creator and Sustainer of all things.

Alvin Plantinga highlights the theological significance of Aquinas’s argument:

“The Second Way offers a rational foundation for understanding God as the necessary being upon which all contingent existence depends. It provides a bridge from reason to faith.”⁷

This reasoning complements biblical theology. In Colossians 1:16-17, Paul writes, “For by Him all things were created… and in Him all things hold together.” Aquinas’s argument provides a philosophical framework for this truth, showing that the very existence of the universe points to God as the ultimate cause.

Responding to Objections

Critics of the Second Way often raise objections rooted in materialism, naturalism, or modern science. Some argue, for instance, that the universe could be eternal and thus require no cause. However, Aquinas’s argument is not about the temporal beginning of the universe but the necessity of a First Cause to sustain any causal chain. As Norman Geisler explains:

“Aquinas’s Second Way does not rest on the assumption that the universe began in time. Instead, it addresses the deeper question of why causation exists at all, whether the universe is eternal or not.”⁸

Others point to quantum mechanics, suggesting that events can occur without a cause. Yet, as Edward Feser notes, this misunderstanding conflates randomness within a system with causality itself:

“Quantum events do not exist in a metaphysical vacuum. They still presuppose a causal framework that must be grounded in something necessary.”⁹

David Bentley Hart critiques the materialist reliance on physical explanations alone:

“To explain the universe in purely physical terms is to fail to explain it at all, for the very principles of causality demand an ultimate source outside the system.”¹⁰

Aquinas’s Second Way addresses the fundamental metaphysical question that naturalism cannot answer: why there is something rather than nothing.

Contemporary Relevance

The Second Way remains profoundly relevant in modern discussions about the existence of God. While science explains the mechanisms of the universe, it cannot account for why those mechanisms exist in the first place. As Peter Kreeft states:

“The Second Way does not compete with science; it completes it. Science tells us how things work, but philosophy tells us why they exist.”¹¹

In an age dominated by naturalistic explanations, Aquinas’s Second Way challenges the assumption that the universe is self-sufficient. It directs attention to the necessity of a transcendent reality that grounds all causation.

Causation and the Divine

Aquinas’s Second Way is a philosophical masterpiece that transcends its medieval origins. By demonstrating the impossibility of an infinite regress of causes, Aquinas leads us to the necessity of a First Cause—a being uncaused, eternal, and self-sufficient. As Aquinas himself wrote, “This first cause is what everyone calls God.”¹²

This argument compels us to recognize that the universe is not self-explanatory. Every effect points to its cause, and every cause ultimately points to God. The Second Way is not merely an abstract proof but an invitation to see the divine reality that undergirds all existence—a truth as relevant today as it was in the 13th century.

¹ Edward Feser, Aquinas: A Beginner’s Guide (London: Oneworld, 2009).

² Thomas Aquinas, Summa Theologica, I, Q. 2, Art. 3.

³ Ibid.

⁴ David Bentley Hart, The Experience of God: Being, Consciousness, Bliss (New Haven: Yale University Press, 2013).

⁵ William Lane Craig, Reasonable Faith (Wheaton: Crossway, 2008).

⁶ Étienne Gilson, The Christian Philosophy of St. Thomas Aquinas (Notre Dame: University of Notre Dame Press, 1994).

⁷ Alvin Plantinga, God and Other Minds (Ithaca: Cornell University Press, 1967).

⁸ Norman Geisler, Thomas Aquinas: An Evangelical Appraisal (Grand Rapids: Baker, 1991).

⁹ Feser, Aquinas: A Beginner’s Guide.

¹⁰ Hart, The Experience of God.

¹¹ Peter Kreeft, Summa of the Summa (San Francisco: Ignatius Press, 1990).

¹² Aquinas, Summa Theologica, I, Q. 2, Art. 3.