Thomas Aquinas

1. Aquinas separates knowledge into natural reason and divine revelation.

For certain things that are true about God wholly surpass the capability of human reason, for instance that God is three and one: while there are certain things to which even natural reason can attain, for instance that God is, that God is one, and others like these, which even the philosophers proved demonstratively of God, being guided by the light of natural reason.

Twofold truth. Thomas Aquinas posits a "twofold truth of divine things": some truths about God are accessible through natural reason (e.g., God's existence, oneness), while others can only be known through divine revelation (e.g., the Trinity). This distinction forms the bedrock of his philosophical theology, suggesting a foundational role for human intellect in understanding God.

Natural reason's scope. Aquinas believes that natural reason, inherent in all humans, can demonstrate God's existence and some of his attributes by observing the created world. He cites Romans 1:19 to support this, interpreting it as evidence that the human intellect can, by itself, arrive at certain divine truths. This "light of natural knowledge" is seen as a universal capacity, allowing philosophers to prove God's existence.

Implicit knowledge. Beyond these two primary modes, Aquinas also acknowledges a vague, "implicit" knowledge of God, present in all humans through their awareness of "being" or their natural desire for happiness. However, this implicit knowledge is considered too general and ambiguous to form a substantial part of his epistemological structure, leaving the "twofold" distinction as his primary focus.

2. Aquinas argues God's existence is not self-evident to humanity.

Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature—namely, by effects.

Self-evidence defined. For Aquinas, a proposition is self-evident if its predicate is inherently contained within its subject (e.g., "Man is an animal"). While "God exists" is self-evident in itself because God's essence is his existence, it is not self-evident to us because humans cannot fully grasp God's essence. Therefore, God's existence must be demonstrated through his effects in the world.

Refuting Anselm. Aquinas rejects Anselm's ontological argument, which claims that the concept of God as "that than which none greater can be conceived" necessarily implies his existence. Aquinas argues that merely having a concept in the mind does not guarantee its reality, and people can hold different, even erroneous, concepts of God (e.g., equating the world with God).

Knowledge through effects. Since direct knowledge of God's essence is beyond human capacity, our understanding of God must be mediated. We come to know God indirectly, through reasoning from his created effects, rather than through an immediate, intuitive grasp of his being. This sets the stage for his famous "Five Ways" to prove God's existence.

3. Aquinas's epistemology is inextricably linked to metaphysics.

The structure of demonstration, then, is isomorphic with the metaphysical structure of reality: immediate, indemonstrable propositions express metaphysically immediate facts, whereas mediate, demonstrable propositions express metaphysically mediate facts.

Foundationalism's depth. Aquinas's epistemology is a form of foundationalism, where knowledge rests on immediate, basic propositions. However, unlike many modern foundationalists, Aquinas insists that these immediate propositions are not merely epistemic grounds but are themselves grounded metaphysically. The truth of what we know depends on the nature of reality itself.

Reality's structure. For Aquinas, which propositions are immediate depends on the "basic structure of the world"—the real natures and relations among them. Justification of knowledge is not solely an epistemological exercise but requires a metaphysical framework where facts, natures, and their constituents exist and are known immediately. This pre-Kantian view sees epistemology and metaphysics as inseparable.

Demonstration mirrors reality. The way we demonstrate knowledge, through syllogistic reasoning from immediate principles, directly reflects the metaphysical structure of reality. Immediate propositions express "metaphysically immediate facts" (relationships between natures and their essential constituents), while mediate propositions express "metaphysically mediate facts." This ensures that our knowledge is rooted in the objective truth of being.

4. Traditional Thomism upholds philosophical "preambles of faith."

The existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles; for faith presupposes natural knowledge, even as grace presupposes nature, and perfection supposes something that can be perfected.

Philosophy's autonomous role. Traditional Thomism, as interpreted by figures like Cardinal Cajetan, asserts that philosophy, operating through natural reason, has an autonomous domain separate from theology. It can establish certain truths about God, known as "praeambula fidei" (preambles of faith), which are necessary rational foundations for understanding revealed theological truths.

Nature precedes grace. This view posits a "pure nature" that can function independently of grace. Natural reason, therefore, can arrive at truths about God's existence and attributes without the aid of supernatural revelation. These philosophical truths serve as a rational substructure upon which the edifice of faith can be built, making theology dependent on philosophy for its initial rational grounding.

Apologetic implications. This framework shapes Aquinas's apologetics, where the existence of God is demonstrated through philosophical arguments accessible to all, regardless of their faith. The authority of philosophers, like Aristotle, is invoked to support these rational demonstrations, which are seen as extrinsic but probable arguments, preparing the mind for the acceptance of revealed truths.

5. Reformed theology critiques natural reason's ability to know God due to sin.

Whereas the medieval doctors had assumed that the fall affected primarily the will and its affections and not the reason, the Reformers assumed also the fallenness of the rational faculty: a generalized or “pagan” natural theology, according to the Reformers, was not merely limited to nonsaving knowledge of God—it was also bound in idolatry.

Sin's radical effect. Reformed theology fundamentally disagrees with Aquinas's optimistic view of natural reason. The Reformers, following Calvin, emphasized the radical impact of sin on all human faculties, including the mind. They argued that fallen human reason is not neutral but is corrupted by depravity, rendering it incapable of arriving at true knowledge of the true God.

Idolatrous conclusions. Any "natural theology" produced by unregenerate reason, according to the Reformers, is "bound in idolatry." The god conceived by such reason is a product of human imagination, not the God of Scripture. This introduces a "soteriological dimension" to epistemology, highlighting the antithesis between the knowledge of believers and unbelievers.

Paul's use of philosophers. The Reformed critique extends to Aquinas's interpretation of Paul's Areopagus address (Acts 17). Aquinas saw Paul's quotation of Aratus ("For we are also His offspring") as an example of natural reason grasping truth. However, Reformed scholars argue Paul recontextualized an idolatrous statement (referring to Zeus) by importing the biblical God, transforming a false proposition into a true one, rather than affirming pagan philosophy's inherent truth.

6. Biblical revelation asserts God's existence is universally self-evident to all.

For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. (Rom. 1:19–20)

God's active revelation. Contrary to Aquinas's view that God's existence is not self-evident to us, Scripture (John 1:9, Romans 1:18-20) teaches that God actively and universally reveals himself to all people. The Logos, the second person of the Trinity, has been "enlightening everyone" since creation, imparting true knowledge of God to all humanity.

Suppression of truth. Romans 1:18-20 emphasizes that God's wrath is revealed against those who "suppress the truth" they already know. This implies that knowledge of God's "invisible attributes"—his eternal power and divine nature—is "clearly perceived" by all. The problem is not a lack of evidence or an inability to reason, but a sinful refusal to acknowledge what is inherently known.

Covenantal knowledge. This universal knowledge is not a vague, generic idea of "being" or "happiness," but a personal, covenantal knowledge of the triune God. It includes an awareness of God's righteous decree and the penalty for disobedience (Rom. 1:32). This knowledge is immediate and independent of human reasoning processes, serving as the basis for universal condemnation.

7. Aquinas's "Five Ways" prove a First Cause, not necessarily the Christian God.

Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.

A posteriori arguments. Aquinas's "Five Ways" (quinque viae) are a posteriori arguments, starting from observable phenomena in the world and reasoning back to a First Cause. They include arguments from:

• Motion/Change (Unmoved Mover)
• Efficient Causality (First Cause)
• Contingency (Necessary Being)
• Degrees of Perfection (Most Perfect Being)
• Governance of the World/Teleology (Intelligent Designer/Director)

Metaphysical underpinnings. These proofs are deeply rooted in Aquinas's metaphysics, particularly his distinction between essence and existence, and his concept of the "analogy of being" (analogia entis). He argues that created things "participate" in existence, which is limited by their essence, implying a pure, unlimited Act of Being (God) as their ultimate cause.

Limitations of natural reason. From a Reformed perspective, the "Five Ways," while logically structured, are ultimately insufficient to prove the existence of the Christian God. Natural reason, even if it could logically deduce a First Cause or Necessary Being, cannot imbue that being with the attributes of the triune, personal, covenantal God of Scripture. The proofs, relying on a supposedly neutral reason, lead to a philosophical god, not the God who reveals himself.

8. Aquinas's divine simplicity is foundational but creates theological tensions.

Now, because we cannot know what God is, but rather what He is not, we have no means for considering how God is, but rather how He is not. . . . Now it can be shown how God is not, by denying of Him whatever is opposed to the idea of Him—viz., composition, motion, and the like.

God as pure actuality. For Aquinas, divine simplicity is a cornerstone of his natural theology, asserting that God is not composed of parts. God is "actus purus" (pure actuality), meaning there is no potentiality in him; his essence is identical with his existence (ipsum esse subsistens). This implies God is complete, independent, and absolute, without any accidents or additions.

Denial of composition. Aquinas systematically denies various forms of composition in God:

• Not a body, nor composed of matter and form.
• No composition of essence and subject, or essence and existence (they are identical).
• Not composed of genus and difference, as God is not a species within a larger category.
• Not composed of subject and accident, as accidents would imply incompleteness or dependence.

Apophatic approach. Aquinas largely approaches divine simplicity through apophatic theology, defining God by what he is not. This method aims to preserve God's absolute uniqueness and transcendence, ensuring that no creaturely limitations or complexities are ascribed to the divine being.

9. Divine simplicity struggles to reconcile God's freedom and responsiveness.

If God can do other than he does, then it is possible for God to exist as God and yet will differently from the way he actually does will. If the nature of God is invariable, God must be the same in all possible worlds in which he exists.

The problem of contingency. A major challenge to divine simplicity is reconciling it with God's freedom and contingent acts, such as creation. If God's essence is identical with his existence and all his properties, then how can he freely choose to create (an act he didn't have to do) without implying a change or distinction within his simple being? This suggests God might have "intrinsic accidental properties," which contradicts simplicity.

God's responsiveness. Similarly, divine simplicity appears to conflict with God's responsiveness to creation and his people, as depicted in Scripture. If God's talking to Cain, for example, is identical with his simple essence, then it seems to be a necessary act, not a free, contingent one. How can a simple, immutable God engage in dynamic, personal interactions without internal change?

Cambridge properties and conditional necessity. Thomists like Eleonore Stump attempt to resolve this by arguing that God's contingent choices result in "Cambridge properties"—extrinsic properties that do not intrinsically affect God's simple nature. They also introduce "conditional necessity," where once God contingently wills an act (e.g., creation), subsequent acts become necessary in that context, without implying intrinsic change in God's will. However, critics argue this still implies real distinctions in God's intrinsic power or will.

10. A Trinitarian-covenantal framework offers a biblical understanding of simplicity.

The Father is simple, the Son is simple, and the Holy Spirit is simple; yet there are not three simples, but one.

Trinity and simplicity. A biblical, Reformed understanding of divine simplicity begins with the ontological Trinity. God's one, simple essence is understood as existing in three distinct hypostases (persons): Father, Son, and Holy Spirit. These ad intra (intrinsic) distinctions and reciprocal acts (e.g., Father begetting the Son) are integral to God's simplicity, not a contradiction of it.

Covenantal condescension. God's freedom and responsiveness are best understood through his eternal decree and the pactum salutis (covenant of redemption). In this eternal counsel, the triune persons freely determine to create and redeem. God's "condescension" in creation and redemption, culminating in the incarnation, involves the persons expressing God's simple essence in distinct, contingent, ad extra ways, without altering his essential character.

Personal, not abstract. This framework moves beyond abstract categories of esse (being) and id quod est (essence) to affirm God as a triune, personal esse. God's one will acts distinctly in each person, allowing for contingent choices and dynamic responsiveness (e.g., grace, wrath) that are consistent with his simple, immutable essence. The incarnation, for instance, is the person of the Son taking on human nature, not a change in the divine essence itself.