Argument from Efficient Causation

1. Introduction

The Argument from Efficient Causation—often called Aquinas’s Second Way—is a classic form of the cosmological argument that infers a first, uncaused cause from the existence of caused things in the world. It focuses specifically on efficient causes, understood in the Aristotelian–Scholastic tradition as the active sources that bring about effects.

Unlike arguments that begin from contingency, motion, or the temporal beginning of the universe, this argument centers on the structure of causal dependence here-and-now. It claims that certain kinds of causal series cannot be infinitely extended without undermining the existence of the very effects we observe. From this, it concludes that there must be a first efficient cause that does not itself depend on any prior efficient cause.

Within the broader landscape of cosmological arguments, the Argument from Efficient Causation is distinctive in several ways:

Philosophers and theologians have interpreted the argument in different ways. Some read it as a timeless argument about metaphysical dependence rather than about temporal beginnings; others assimilate it to broader first-cause reasoning about the origin of the cosmos. Contemporary discussions often reformulate the argument using modern notions of causation, necessity, and explanation.

Supporters present it as a rigorous, deductive argument whose validity is relatively uncontroversial, while treating its soundness—whether its premises are true—as a subject of ongoing debate. Critics challenge key assumptions about causation, the impossibility of infinite regress, and the move from a first cause to a theistic God.

The following sections examine the historical emergence of the Argument from Efficient Causation, its classical formulation in Aquinas, its logical form, the main premises and their justifications, prominent objections, and its role in contemporary philosophy of religion and metaphysics.

2. Origin and Attribution

The Argument from Efficient Causation is most closely associated with Thomas Aquinas (1224/5–1274), who presents it as the Second Way in Summa Theologiae, I, q.2, a.3. While Aquinas is its canonical formulator, the argument draws on a long tradition of reflection on causality.

Intellectual Lineage

Aquinas adapts and systematizes themes from:

Aquinas integrates these traditions within a Scholastic framework, using Aristotelian concepts while engaging Islamic and Christian predecessors.

Attribution and Naming

The label “Second Way” derives from Aquinas’s own organization of five brief arguments for God’s existence. The more descriptive titles—“Argument from Efficient Causation” or “Argument from Efficient Causes”—are modern, especially in Anglophone scholarship.

Some historians emphasize that Aquinas’s Second Way is not simply a generic “first-cause” argument but a specifically per se causal series argument about present dependence. Others group it together with broader cosmological arguments under headings such as “the second cosmological argument.”

While Aquinas is generally regarded as the principal architect of the argument in its classical form, later Thomists and neo-Scholastics have significantly developed its details. Alternative attributions sometimes highlight Avicenna or Leibniz as offering structurally related arguments from causal or explanatory dependence, though these are typically classified separately (e.g., as arguments from contingency or sufficient reason).

3. Historical Context and Scholastic Background

The Argument from Efficient Causation emerges within the High Medieval period, when Western Christian thinkers were assimilating Aristotle’s works and interacting with Islamic and Jewish philosophy. Aquinas writes in a milieu shaped by Scholastic methods: rigorous disputation, fine-grained conceptual distinctions, and systematic synthesis of philosophy and theology.

Aristotelian Revival and Four Causes

The 12th–13th centuries saw a large-scale recovery of Aristotle’s corpus, often via Arabic and Latin translations. This brought into Latin Christendom:

• A structured theory of four causes (material, formal, efficient, final).
• Analyses of change, motion, and potency/act.
• Discussion of whether causal chains can be infinite.

Scholastics treated efficient causality as central for explaining change and existence, preparing the conceptual ground for Aquinas’s Second Way.

Debates on Eternity and Creation

A key background issue was whether the world is eternal or has a temporal beginning. Aristotle held that motion and the world are eternal; Christian doctrine affirmed creation ex nihilo. Aquinas maintained that, although revelation teaches a beginning in time, philosophical demonstration cannot decisively prove or disprove an eternal universe.

Within this framework, arguments like the Second Way were not designed primarily to show a temporal first moment but to establish a metaphysical first cause even if the world were eternal. This shaped the emphasis on essentially ordered causal series, which concern hierarchical dependence rather than chronological succession.

Scholastic Method and Causation

The Scholastic method involved quaestiones (questions) structured around objections, sed contra authorities, and responsiones (solutions). Within this practice, causal analysis became a key tool both in natural philosophy and in natural theology.

Different schools—Augustinians, Thomists, Scotists, and later nominalists—developed competing accounts of causation:

The Argument from Efficient Causation presupposes an Aristotelian–Thomist conception of causality, which later critics and alternative traditions would challenge or reinterpret.

4. Aquinas’s Second Way in the Summa Theologiae

In Summa Theologiae I, q.2, a.3, Aquinas presents five ways of proving God’s existence. The Second Way is his concise formulation of the Argument from Efficient Causation.

Textual Presentation

Aquinas’s Latin text, in translation, runs only a few paragraphs. A representative portion states:

"

In the world of sense we find there is an order of efficient causes. There is no case known (nor indeed, is it possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible.
Now in efficient causes it is not possible to go on to infinity... Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.

— Thomas Aquinas, Summa Theologiae I, q.2, a.3

The structure is brief and compressed, assuming background familiarity with Aristotelian metaphysics. Later Thomists and commentators have expanded and clarified what they take to be Aquinas’s implicit reasoning.

Place Among the Five Ways

The Second Way appears immediately after the First Way (from motion) and before the Third Way (from possibility and necessity). Commentators differ on how distinct these ways are:

• Some argue the Second Way is largely an alternative formulation of the First, now in terms of efficient causation rather than motion.
• Others maintain that it targets a different feature of reality—productive causation—and gives an argument that does not depend on motion.

Regardless, Aquinas clearly intends the Second Way to be an independent path that begins from empirical observation of caused things.

Intended Function

Within the Summa, the Second Way functions as part of a preliminary demonstration that God exists before Aquinas proceeds to questions about the divine attributes. It is designed to be:

• A posteriori (starting from sensory experience).
• Brief and schematic, suitable for a theological manual rather than an extended philosophical treatise.
• Compatible with the possibility of an eternal universe, focusing on causal dependence rather than temporal origin.

Subsequent scholastic and modern interpreters have elaborated the Wayne’s assumptions and implications, but the Summa itself offers only this succinct statement.

5. The Argument from Efficient Causation Stated

While Aquinas states the Second Way compactly, later presentations standardly unpack it into a more explicit argument about efficient causes and causal series.

Standard Formulation

A commonly used reconstruction, consistent with the reference data, runs as follows:

1. In the world of sense we find an order of efficient causes; some things are efficiently caused.
2. Nothing can be the efficient cause of itself, because that would require it to exist prior to itself.
3. In an essentially ordered (here-and-now dependent) series of efficient causes, an infinite regress is impossible; otherwise there would be no first cause.
4. If there is no first efficient cause in such an order, there would be no intermediate or ultimate causes, and hence no effects, contrary to what we observe.
5. Therefore, there must be a first efficient cause that is not itself caused by anything else.
6. This first uncaused efficient cause is what people call God.

Key Features of the Statement

• The starting point is empirical: the existence of caused things and observable causal relations.
• The argument employs a restriction to a particular type of series, often called essentially ordered or per se.
• The central move is from the impossibility of infinite regress in such a series to the existence of a first cause.
• The final step is a terminological or identificatory claim: that such a first cause is what is meant by “God,” which later sections examine in more detail.

Alternative formulations may adjust the wording of premises (e.g., replacing “everything has a cause” with “everything contingent has a cause”), but they typically preserve this overall structure: from caused effects, via rejection of an infinite per se regress, to a first uncaused cause.

6. Logical Structure and Deductive Form

Philosophers generally interpret the Argument from Efficient Causation as a deductive argument: if its premises are true and its inferences valid, its conclusion follows with necessity.

Formal Structure

A simplified logical schema, abstracting from theological language, can be represented as:

1. There exist effects that are efficiently caused.
2. For any effect E in a certain kind of causal series S, E’s existence here-and-now requires an efficient cause C that is prior in S.
3. No member of S can be the efficient cause of itself.
4. S cannot be infinite in the relevant “prior–posterior” sense.
5. Therefore, S has a first member F that is not caused by any prior efficient cause in S.
6. Any such F must exist and cause in a way that does not depend on any further efficient cause.
7. Therefore, there exists at least one uncaused first efficient cause.

The move from (1–4) to (5) is often treated as purely logical, given an understanding of “ordered series” and “infinite.” The controversial issues concern the truth and interpretation of the premises, not so much the inferential pattern.

Validity Considerations

Analytic philosophers often ask whether the argument is valid in standard logic. Many defenders argue that, once the relevant concepts (e.g., “essentially ordered series”) are defined, the inference from “no infinite regress of such causes” to “there is a first cause” is straightforwardly valid.

Skeptics sometimes claim that the argument smuggles in additional assumptions—for instance, about the transitivity of dependence or the necessity of a well-founded structure of explanation. In response, proponents typically formulate the argument within a more explicit framework (e.g., well-foundedness in set theory or partial orders) to show that:

• If a non-empty partially ordered set has no infinite descending chain,
• Then it must have at least one minimal element (analogous to a first cause).

Under such formalizations, the argument is usually regarded as logically valid given its stated premises.

Deductive vs. Explanatory Reading

Some contemporary authors suggest reading the argument not only as a deductive chain but also as an argument about explanatory structure: that only a terminating, non-derived cause can ultimately account for the existence of derivative causes and their effects. This does not alter its deductive ambitions but highlights its metaphysical assumptions about what counts as an adequate explanation.

7. Efficient Causality and Types of Causal Series

A central feature of the Argument from Efficient Causation is its reliance on a specific conception of efficient causality and its distinction between different types of causal series.

Efficient Cause in the Aristotelian–Scholastic Sense

In this tradition, an efficient cause is the agent or source that brings about an effect or change. It is contrasted with:

Efficient causes are typically understood as real agents whose causal powers are grounded in their natures and can be acted upon or derived from other agents.

Essentially Ordered vs. Accidentally Ordered Series

Aquinas and later Thomists distinguish two important kinds of causal series:

In an essentially ordered (per se) series, the whole chain is said to be causally active only insofar as the first member is currently imparting causal efficacy. Remove the first, and the rest lose their causal power. In an accidentally ordered (per accidens) series, by contrast, once a cause has produced its effect (e.g., a parent producing a child), the effect can itself become a cause independently.

Significance for the Second Way

The Second Way primarily targets essentially ordered efficient-causal series. Aquinas’s claim that an infinite regress in efficient causes is impossible is usually interpreted as restricted to such here-and-now dependence. The argument does not purport (on most interpretations) to rule out infinite temporal sequences of causes of the per accidens kind; rather, it focuses on whether there can be an infinite hierarchy of derivative causal powers without an ultimate source.

This typology of causal series is therefore not a mere taxonomic detail; it is structurally embedded in how proponents understand the core premises of the argument.

8. Premises Examined and Justified

Each premise of the Argument from Efficient Causation has been subject to detailed analysis and dispute. Interpretations often differ over how strong each claim needs to be.

Premise 1: Existence of Efficient Causes

The claim that “in the world of sense we find an order of efficient causes” is typically taken as an empirical observation: things change, begin to exist, and are produced by other things.

• Proponents treat this as minimally contentious, pointing to everyday examples (artifacts, biological reproduction, mechanical processes).
• Some philosophers with causal skepticism question whether we directly observe causation or only regular succession, following a broadly Humean line.

Premise 2: Nothing Is the Efficient Cause of Itself

This premise asserts that self-causation would require a thing to be prior to itself, which is taken to be impossible.

• Defenders see this as almost analytic, grounded in the temporal or ontological asymmetry between cause and effect.
• A minority of philosophers have explored exotic models (e.g., certain spacetime structures or causal loops) in which some form of self-causation might be coherent, though these are controversial and often not straightforwardly applicable to Aquinas’s notion of efficient cause.

Premise 3: Impossibility of Infinite Regress in Essentially Ordered Series

This is the most contested premise. Aquinas’s brief assertion that infinite regress is impossible has been variously interpreted:

• Thomists argue that in an essentially ordered series, each member has only derivative causal power; without a first, there would be no source of causal efficacy.
• Critics contend that the mere existence of an infinite sequence of derived powers need not be incoherent, questioning whether well-foundedness is required.

Detailed arguments for and against this premise are examined in the dedicated section on infinite regress.

Premise 4: No First, No Intermediates, No Effects

Aquinas holds that if there were no first cause in an essentially ordered series, there could be no middle causes and thus no effects.

• Proponents understand this as a claim about ontological dependence: derivative causes cannot function without a non-derivative source.
• Objectors sometimes argue that the inference from “each depends on a prior” to “the whole has a first” risks a form of composition reasoning, though defenders dispute this characterization.

Conclusion Premises

The move from “first efficient cause” to “uncaused” is usually seen as definitional: being first in the relevant order means not having a prior efficient cause. The identification of such a first cause with God involves additional reasoning about the attributes such a cause must have, which is treated separately in the section on “From First Cause to God.”

9. The Rejection of Infinite Regress

A central pillar of the Argument from Efficient Causation is the claim that an infinite regress of efficient causes of a certain kind is impossible. The debate focuses on what sort of impossibility is at stake and what kind of infinite series is being excluded.

Aquinas’s Claim

Aquinas states that “in efficient causes it is not possible to go on to infinity,” linking this to the idea that without a first cause there would be neither intermediates nor effects. Later interpreters generally construe this as:

• A restriction to essentially ordered (per se) series.
• A claim about metaphysical (not merely epistemic or practical) impossibility.

Standard Thomist Rationale

Defenders often reason as follows:

1. In an essentially ordered series, each later cause has no independent causal power; its efficacy is wholly derived from earlier causes in the series.
2. If there were no first, non-derived source of causal power, there would be nothing from which this power could be derived.
3. Therefore, an infinite regress of purely derivative causes would leave the series with no causal efficacy at all, contradicting the existence of actual effects.

Analogies include a chain hanging from a hook: infinitely many links with no hook would, on this view, not hang at all.

Alternative Evaluations of Infinite Regress

Philosophers categorize regresses as:

Proponents argue that an infinite per se series would be vicious, since it would never ground causal power. Critics counter that:

• Infinite regresses in other contexts (e.g., numbers, some physical models) are coherent, and
• It is not evident that derived causal powers cannot be infinitely many without a foundational source.

Some suggest that a brute fact about the existence of an infinite causal network might be acceptable, obviating the need for a first cause.

Logical and Metaphysical Frameworks

Modern metaphysicians sometimes recast the issue in terms of well-foundedness of dependence relations:

• Proponents claim that dependence or grounding relations are, by their nature, well-founded, excluding infinite descending chains.
• Opponents challenge this as a substantive metaphysical assumption, not something self-evident.

Thus, the rejection of infinite regress in the Second Way is widely regarded as a non-trivial and heavily contested premise.

10. From First Cause to God

The Argument from Efficient Causation, as classically stated, concludes with a first efficient cause and then identifies this with God. This identification is not immediate; it involves further reasoning about what such a cause must be like.

Minimal Conclusion of the Second Way

On a conservative reading, the argument’s minimal metaphysical output is:

• There exists at least one uncaused first efficient cause in essentially ordered series of efficient causes.

By itself, this does not specify that the cause is unique, omnipotent, omniscient, or morally perfect.

Aquinas’s Broader Project

Aquinas does not attempt to derive the full doctrine of God within the Second Way itself. Instead, he treats the Ways as establishing that something worthy of the name “God” exists, then dedicates subsequent questions in the Summa Theologiae (e.g., q.3–q.26) to analyzing the divine attributes.

The general strategy is:

1. From first-cause status, infer features like uncaused, necessary, non-composite, and pure act (no unrealized potentials).
2. From these features, further derive traditional classical theist attributes: simplicity, immutability, eternity, omnipotence, and so forth.

Thus, the identification “and this everyone calls God” signals the start of a longer metaphysical development rather than its endpoint.

Interpretive Perspectives

Different traditions interpret the move from first cause to God in various ways:

Some contemporary proponents explicitly separate the argument into two stages: (1) from caused things to a first cause; (2) from first cause to a God-like being, with the second stage requiring additional arguments about the explanatory role and modal status of the first cause.

The Second Way itself, however, only sketches this transition by appealing to a shared, pre-philosophical notion of “God” as the ultimate source of all other things.

11. Key Variations and Related Cosmological Arguments

The Argument from Efficient Causation belongs to a broader family of cosmological arguments, many of which share overlapping intuitions but differ in formulation and emphasis.

Medieval and Early Modern Variants

Subsequent thinkers adapted the Second Way in diverse directions:

These arguments often intersect with efficient-cause reasoning but are typically classified as contingency or sufficient-reason arguments rather than pure first-cause arguments.

Modern Cosmological Arguments

In contemporary philosophy of religion, several prominent forms are often distinguished:

The Thomistic Second Way is frequently contrasted with the kalam argument: it can, on many readings, allow for an eternal universe, focusing instead on hierarchical dependence.

Internal Variants of the Second Way

Within Thomist and neo-Scholastic traditions, variations arise concerning:

• How strictly to distinguish per se and per accidens series.
• Whether the first cause is conceived as primarily an existence giver (conserving cause) or as an initiator of processes.
• How to formulate the causal principle (e.g., “whatever is moved is moved by another” vs. “whatever is contingent has a cause”).

Some analytic philosophers recast the Second Way using contemporary notions of grounding, ontological dependence, or explanatory priority rather than traditional efficient-cause vocabulary, while claiming continuity of underlying structure.

These related arguments and variants illustrate how the central intuition—about the insufficiency of derivative causes without a first source—has been expressed in multiple metaphysical idioms.

12. Major Objections and Critiques

The Argument from Efficient Causation has faced a range of objections, targeting both its premises and its conclusions. Several have become standard in the literature.

Objection: Infinite Regress May Be Possible

Philosophers such as William L. Rowe and J. L. Mackie argue that Aquinas does not adequately demonstrate the impossibility of an infinite regress of efficient causes, even in an essentially ordered sense. They suggest that:

• An infinite regress of dependent causes might be metaphysically coherent.
• The mere fact that each cause has a prior need not entail that the whole has a first cause.

This objection questions whether the regress in question is genuinely vicious rather than benign.

Objection: Fallacy of Composition

Bertrand Russell and Graham Oppy, among others, contend that the argument risks a fallacy of composition: inferring from the fact that each member of a series has a cause that the series as a whole must have a single external cause. They argue that:

• Even if every event in the universe has a cause, it does not follow that the universe itself has a cause.
• The explanation of each part might suffice for explaining the whole.

Defenders reply that the Second Way focuses on the structure of dependence within a single ordered series, not on aggregating part–whole properties.

Objection: Why Exempt God from Causal Principles?

Critics from Hume to Mackie argue that the argument appears to apply a causal principle universally, then makes an exception for God:

• If everything requires a cause, why does God not?
• If one can accept an uncaused God, why not accept an uncaused universe or brute causal order instead?

Proponents often respond by refining the causal principle (e.g., applying it only to contingent or composite beings), but critics may view such refinements as ad hoc.

Objection: Dependence on Aristotelian–Scholastic Metaphysics

Some, like Anthony Kenny and Jordan Howard Sobel, question the argument’s reliance on Aristotelian concepts:

• Essentially ordered series and robust efficient causation may not align with modern science or metaphysics.
• If the underlying metaphysical framework is rejected, the premises lose their force.

This raises the issue of whether the Second Way can be translated into contemporary metaphysical language without loss.

Objection: Alternative Explanatory Principles

Others suggest that appeals to brute facts, probabilistic explanations, or non-causal forms of explanation might render a first cause unnecessary. For example:

• The existence of the causal network might be a brute fact.
• Or it might be explained by non-personal laws or principles rather than by a First Cause in any robust agent-like sense.

These critiques challenge whether the argument’s conclusion is needed, even if its premises are granted.

13. Contemporary Thomist and Analytic Responses

Recent Thomist and analytic philosophers have developed sophisticated responses to the standard objections, often updating the Second Way’s framework.

Refining the Causal Principle

Contemporary defenders frequently reject the simplistic claim that “everything has a cause.” Instead, they adopt more nuanced principles such as:

• “Whatever is contingent or has a real distinction between essence and existence has a cause.”
• “Whatever has derived causal power requires a source.”

This is meant to avoid the charge of special pleading for God by defining God as a being whose existence and causal power are non-derived.

Emphasis on Essentially Ordered Series and Sustaining Causation

Authors like Edward Feser and Brian Davies stress that the Second Way concerns here-and-now sustaining causation:

• The first cause is not merely the earliest cause in time but a concurrent sustaining cause of the existence and activity of all derivative causes.
• This is sometimes re-framed using the contemporary notion of grounding or ontological dependence rather than temporal sequence.

On this view, an infinite past would not refute the argument, since the relevant causal order is simultaneous and hierarchical.

Responses to Infinite Regress Objections

To defend the rejection of infinite regress, contemporary Thomists often:

• Appeal to the idea that certain dependence relations are well-founded by their nature.
• Use analogies from mathematics (e.g., well-founded orders) or metaphysics (e.g., grounding chains) to argue that derivative entities must terminate in something non-derivative.

Opponents reply that well-foundedness remains a substantive assumption, but defenders treat it as a plausible metaphysical thesis.

Addressing Composition and Brute Fact Concerns

In reply to composition objections, Thomists emphasize that the argument does not move from “each part has a cause” to “the whole has a cause,” but from the internal structure of a single causal chain to the existence of a non-derivative member.

Regarding brute facts, many analytic theists claim that positing a necessary first cause offers a deeper, more unified explanation than accepting unexplained brute existence of the causal order. Critics question whether this explanatory gain is sufficient or whether brute facts are ultimately unavoidable.

Engagement with Analytic Metaphysics

Some analytic philosophers sympathetic to the Second Way recast it in terms of:

• Grounding: insisting that grounding chains must terminate in a fundamental ground.
• Existential dependence: arguing that derivative existents presuppose a self-existent being.
• Modal metaphysics: presenting the first cause as a metaphysically necessary being.

These reformulations seek to preserve the argument’s core intuition while making it more accessible within contemporary analytic discourse.

14. Relation to Modern Science and Metaphysics of Causation

The Argument from Efficient Causation raises questions about its compatibility with modern science and contemporary theories of causation.

Interaction with Modern Physics

Proponents and critics debate how the argument relates to physical cosmology:

• Some defenders claim the Second Way is largely independent of empirical cosmology, since it concerns metaphysical dependence rather than temporal origins or specific physical processes.
• Others see potential tension with models involving closed timelike curves, quantum indeterminacy, or cosmologies without a clear initial state, which may complicate classical pictures of ordered causation.

Quantum mechanics, with its probabilistic descriptions and debates over whether events occur without deterministic causes, has led some philosophers to question the universality of traditional causal principles. Supporters of the Second Way often respond that:

• The argument depends on a metaphysical notion of efficient causation broader than deterministic physical causes.
• Indeterministic processes may still presuppose some underlying powers or dispositions that function as efficient causes.

Contemporary Metaphysics of Causation

Modern metaphysical theories of causation include counterfactual, regularity, process, power-based, and interventionist accounts. Their relation to the Second Way is complex:

Defenders who adopt powers-based metaphysics often see strong continuity between Aristotelian efficient causes and modern dispositional properties.

Scientific Naturalism and Brute Causality

Some philosophers of science hold that the universe’s causal structure is a brute fact not requiring deeper explanation. From this standpoint:

• The search for a first cause may be seen as methodologically unnecessary or beyond the remit of science.
• The explanatory standards employed by natural science do not typically demand ultimate metaphysical grounding.

Others argue that scientific inquiry leaves open questions about why there is a law-governed causal order at all, inviting metaphysical arguments like the Second Way to address these broader issues.

Independence and Complementarity

Many contemporary Thomists emphasize that the Second Way:

• Does not derive its premises from specific empirical theories.
• Is compatible with multiple scientific cosmologies, provided there is some causal order to analyze.
• Operates at a level of metaphysical explanation that is neither validated nor invalidated straightforwardly by empirical data.

Critics remain divided on whether this separation protects the argument from scientific challenge or renders it insulated and less empirically relevant.

15. Assessment of Validity and Soundness

Philosophers typically distinguish between the argument’s validity (logical structure) and its soundness (truth of premises).

Validity

Many analyses, including those by critics, acknowledge that once the relevant concepts are clarified, the Second Way’s core inference pattern is logically respectable:

• If there exist effects in an essentially ordered series,
• And if no member causes itself,
• And if an infinite regress of such causes is impossible,
• Then a first, uncaused efficient cause follows.

When formalized in terms of partially ordered sets or well-founded relations, the step from “no infinite descending chain” to “existence of a minimal element” is mathematically and logically valid. Disagreements center more on whether the premises are acceptable than on the argument’s overall deductive form.

Soundness: Key Points of Contention

The argument’s soundness is widely regarded as an open question. Major points of dispute include:

1. Causal Premise: Whether the world indeed contains the kinds of efficient-causal relations and essentially ordered series that Aquinas posits.
2. Anti-Self-Causation Premise: Whether self-causation in any relevant sense is impossible.
3. Anti-Infinite-Regress Premise: Whether infinite regress in per se causal series is metaphysically impossible or merely unproven.
4. Explanatory Principle: Whether derivative causes really require a non-derivative source or whether explanations can be non-terminating.
5. Identification Premise: Whether a first cause must have the properties traditionally ascribed to God, or could be something less theologically robust.

Divergent Assessments

Some contemporary philosophers adopt a modest stance: even if the argument does not compel assent, they treat it as a serious, defensible attempt to articulate how metaphysical dependence might lead to a first cause. Others view it as deeply tied to an outdated metaphysics.

Overall, consensus on validity is relatively higher than on soundness; the latter remains a subject of active and nuanced philosophical disagreement.

16. Legacy and Historical Significance

The Argument from Efficient Causation has exerted substantial influence on the history of philosophy, theology, and philosophy of religion.

Medieval and Early Modern Influence

In the centuries following Aquinas, the Second Way shaped:

• Scholastic theology: It became a standard component of Thomist accounts of natural knowledge of God and influenced debates among Dominicans, Jesuits, and other orders.
• Catholic doctrine and education: Thomist cosmological reasoning, including the Second Way, was widely taught in seminaries and universities, especially after the neo-Thomist revival of the 19th–20th centuries.

Early modern philosophers engaged with related causal arguments:

Modern and Contemporary Philosophy of Religion

In the 20th and 21st centuries, the Second Way and its relatives have remained central topics in analytic philosophy of religion:

• Defenders (e.g., Edward Feser, Brian Davies, Norman Kretzmann) have elaborated neo-Thomist versions with updated metaphysical tools.
• Critics (e.g., J. L. Mackie, William Rowe, Graham Oppy) have used the argument as a focal point for examining the limits of cosmological reasoning.

The argument has also influenced systematic theology, where it is often integrated into broader accounts of natural theology and the rationality of theism.

Role in Contemporary Debates

The Argument from Efficient Causation continues to feature in discussions of:

• The metaphysics of causation and grounding.
• The appropriateness of metaphysical explanation in addition to scientific explanation.
• The intellectual viability of classical theism versus alternative conceptions of ultimate reality (e.g., naturalistic or pantheistic views).

Even where philosophers reject the argument, it frequently serves as a paradigm case for analyzing issues about infinite regress, necessary beings, and the structure of explanation.

Educational and Cultural Significance

In philosophy curricula, Aquinas’s Second Way is often one of the first historical arguments for God’s existence that students encounter. It:

• Provides a window into Scholastic methodology and Aristotelian metaphysics.
• Functions as a touchstone for learning to distinguish validity from soundness and to evaluate metaphysical premises.
• Continues to inform public and interfaith discussions about whether the existence of the cosmos calls for a first cause.

Thus, regardless of its ultimate philosophical status, the Argument from Efficient Causation remains a historically central and conceptually rich contribution to debates about causality, existence, and the possibility of a supreme explanatory principle.