Aquinas' Five Ways

Aquinas’ Five Ways are five arguments for God’s existence presented by Thomas Aquinas in his Summa Theologica around 1274, forming the bedrock of natural theology in the Catholic philosophical tradition. Each argument begins with an observable feature of the world - motion, causation, contingency, gradation, or purposeful order - and reasons backward to a divine source. They remain the most widely taught proofs for God in Western philosophy, but modern science and logic have exposed significant weaknesses in all five. We score them 15/100 for soundness.

The First Way - The Argument from Motion

Aquinas observed that everything in the world is in motion (or, more precisely, in a state of change). Whatever is in motion must have been set in motion by something else, since nothing can move itself from potentiality to actuality without an external cause. This chain of movers cannot extend infinitely, Aquinas argued, because without a first mover, no subsequent motion would ever begin. Therefore, there must be an Unmoved Mover - a being that causes all motion without itself being moved - and this is what we call God.

The argument draws heavily on Aristotelian metaphysics, particularly the distinction between potentiality and actuality. For Aquinas, change is the transition from potential to actual, and every such transition requires a cause already in a state of actuality. The First Way is not primarily about temporal sequence (one thing happening after another) but about ontological dependence - what sustains change at every moment.

The Second Way - The Argument from Efficient Causes

The Second Way parallels the First but focuses on causation rather than motion. Every effect in the world has a cause, and that cause itself has a prior cause. An infinite regress of causes is impossible, because without a first cause, no subsequent causes (and hence no effects) would exist. Therefore, a First Efficient Cause must exist - and this is God.

This argument is structurally similar to the Kalam Cosmological Argument, though Aquinas was less concerned with the beginning of time and more with the ongoing causal structure of reality. Modern defenders sometimes distinguish between a temporal series (cause before effect in time) and a hierarchical series (a simultaneous chain of dependence), arguing that only the latter is truly what Aquinas had in mind.

The Third Way - The Argument from Contingency

The Third Way observes that things in the world are contingent - they come into existence and pass away, meaning they could possibly not exist. If everything were merely contingent, then at some point nothing would have existed, and since something cannot come from nothing, there would be nothing now. Since things do exist, there must be at least one necessary being whose existence is not contingent on anything else.

This argument was later refined by Leibniz into a more rigorous form using the Principle of Sufficient Reason. Critics point out that Aquinas’s version contains a logical gap: it does not follow from “each thing could fail to exist” that “all things could fail to exist simultaneously.” The fallacy of composition - attributing a property of individual members to the whole - weakens this particular formulation.

The Fourth Way - The Argument from Degrees

The Fourth Way is widely considered the weakest of the five. Aquinas observed that things exhibit varying degrees of qualities - some things are more good, more true, more noble than others. Degrees of a quality, he argued, require a maximum that serves as the standard and cause of that quality in all other things. Therefore, there must be a being of maximum goodness, truth, and nobility - and this is God.

The reasoning draws on Platonic metaphysics and the idea that degrees imply a perfect exemplar. However, the analogy breaks down under scrutiny. We can rank objects by temperature without needing a “maximally hot” being. Degrees of height do not require a tallest-possible entity. The argument conflates comparative judgments (which are relative) with the existence of an absolute standard, and most contemporary philosophers find this move unjustified.

The Fifth Way - The Argument from Design

The Fifth Way observes that natural bodies act toward ends - acorns grow into oaks, planets follow regular orbits, ecosystems maintain balance. Since unintelligent things cannot direct themselves toward goals, something intelligent must be directing them, just as an arrow requires an archer. This intelligent director of all natural things is God.

This is Aquinas’ version of the teleological argument, and it was perhaps the most intuitively compelling of the Five Ways for centuries. However, it has been the most thoroughly undermined by modern science. Darwin’s theory of natural selection demonstrated that the appearance of purposeful design in biology can arise from undirected evolutionary processes - no intelligent archer required. The regularity of planetary orbits is explained by gravitational mechanics, not divine direction.

Modern Scientific Challenges

Quantum mechanics poses a fundamental challenge to the first two Ways. At the subatomic level, events such as radioactive decay appear to be genuinely uncaused - a radium atom decays at a random moment with no prior determining cause. Virtual particles spontaneously appear and disappear in the quantum vacuum. These phenomena undermine Aquinas’ assumption that every change requires an external cause and every effect requires a prior efficient cause.

The rejection of actual infinities is another vulnerable premise. Aquinas assumed that an infinite causal chain is impossible, but modern mathematics routinely handles infinite sets and series. Some cosmological models, including certain eternal inflation scenarios, posit an infinite past with no first moment. If infinite causal chains are coherent, the need for a “first” cause or mover dissolves.

Additionally, the laws of thermodynamics and conservation of energy provide explanations for change and motion that do not require an external sustaining cause in the way Aquinas envisioned. Energy transforms from one form to another according to physical laws, without needing a concurrent divine mover at each step.

The Gap Between First Cause and God

Even if one or more of the Five Ways succeeded in establishing a first cause, unmoved mover, or necessary being, a significant conceptual gap remains between that conclusion and the God of classical theism. Critics note that the arguments establish (at most) the existence of something that initiated or sustains the causal order. They do not establish that this something is:

• Personal - having consciousness, will, or intentions
• Singular - there could be multiple necessary beings or first causes
• Omnipotent or omniscient - a first cause need not be all-powerful
• Benevolent - nothing in the arguments addresses goodness or love
• Interested in human affairs - an unmoved mover could be entirely indifferent

Aquinas was aware of this gap and addressed it in subsequent sections of the Summa Theologica, but the Five Ways themselves do not bridge it. This is a common criticism of cosmological arguments generally - they may get you to a first cause, but the leap from first cause to a personal, loving God requires additional arguments.

Defenders and Modern Reformulations

The Five Ways are not without contemporary defenders. Thomistic philosophers like Edward Feser argue that most criticisms rest on misunderstandings - particularly the conflation of Aquinas’ hierarchical causation with temporal causation. On this reading, the arguments are not about what happened at the beginning of time but about what sustains existence right now. Every moment of change, they argue, requires a concurrent cause that is itself unchanged.

Other defenders reformulate the arguments using modern metaphysics. The Kalam Cosmological Argument can be seen as a refined version of the Second Way, incorporating Big Bang cosmology to argue that the universe had a temporal beginning and therefore requires a cause. The fine-tuning argument updates the Fifth Way by pointing to the precise physical constants required for a life-permitting universe rather than biological design.

However, even sympathetic philosophers acknowledge that the Five Ways require substantial supplementary argument to reach the conclusion Aquinas intended - the existence of the God described in Christian theology.

Historical Significance

Whatever their logical status, the Five Ways are historically unparalleled in their influence. They shaped Catholic philosophy for over seven centuries and were formally endorsed by the First Vatican Council in 1870 as demonstrating that God’s existence can be known through natural reason. The Summa Theologica remains the central text in Catholic seminary education, and the Five Ways are typically the first arguments students encounter in philosophy of religion courses worldwide.

Their influence extends well beyond Catholicism. Islamic philosophy had developed similar cosmological reasoning centuries earlier through thinkers like Al-Ghazali and Avicenna, and Aquinas explicitly drew on these sources. The arguments also provoked some of the most important work in secular philosophy, from Hume’s critiques of causation to Kant’s analysis of the limits of pure reason.

Our Scoring

Soundness: 15/100. The Five Ways rely on Aristotelian metaphysics and medieval assumptions about physics that have been largely superseded. Quantum mechanics undermines the claim that every change requires an external cause. Evolution by natural selection explains apparent biological design without a designer. The rejection of infinite causal chains is no longer philosophically secure. The Fourth Way (degrees of perfection) commits a logical fallacy. While Thomistic reformulations address some objections, they require substantial additional premises that are themselves contested. Most contemporary philosophers of religion do not consider the original Five Ways sound as stated.

Personal God: 40/100. Even if the arguments succeeded, they establish only an unmoved mover, first cause, or necessary being - none of which is necessarily personal, conscious, or interested in human affairs. Aquinas needed extensive additional argumentation in the Summa to connect the Five Ways to a personal God. The gap between “first cause” and “loving, personal deity who answers prayers” is wide, keeping this score moderate.

Creator/Designer: 65/100. The Five Ways map more naturally onto a creator or designer concept. An unmoved mover that initiates all change, a first cause that originates the causal chain, and an intelligent director of natural ends all fit the idea of a being that created or designed reality. The Fifth Way explicitly invokes design. If any of these arguments worked, a creator or designer is a reasonable characterization of what they establish.

Higher Power: 70/100. This is the highest score because the Five Ways most naturally point to something like a higher power - a transcendent force or necessary being that underlies reality. A necessary being, an unmoved mover, or a first cause all qualify as a “higher power” without requiring the additional attributes (personality, benevolence, omniscience) that the arguments do not establish. The less specific the god concept, the better the Five Ways support it.