On Being and Essence

1. Introduction

De Ente et Essentia (On Being and Essence) is a short Latin treatise, composed by Thomas Aquinas in the 1250s, that offers a compact exposition of some of his most fundamental metaphysical theses. It addresses two closely related questions: what it means for something to be (ens), and what it is that makes a thing the kind of thing it is—its essence (essentia or quidditas).

Within the medieval university curriculum, the text functioned as a metaphysical primer for students already trained in Aristotelian logic and natural philosophy. It presupposes familiarity with Aristotle’s ten categories, the distinction between substance and accident, and the basic framework of act and potency. Against that background, Aquinas seeks to clarify:

• the different senses in which “being” is said;
• how essences are found in material and immaterial entities;
• how essence and existence (esse) relate in created things and in God.

The treatise’s six chapters progress from logical and semantic clarifications to increasingly metaphysical claims about the structure of reality. A central feature is the thesis that, in all finite beings, what a thing is (its essence) is distinct from that it is (its act of existence), whereas in God alone essence and existence are identical. This framework underlies Aquinas’s later accounts of causality, participation, and the nature of God.

Scholars have often treated On Being and Essence as a privileged entry-point into Thomistic metaphysics. Some interpret it as an early, programmatic statement of Aquinas’s mature views; others see it as a transitional work, reflecting his engagement with Aristotle, Avicenna, and contemporary scholastic debates. Despite its brevity, it remains a focal text for discussions of medieval ontology, the metaphysics of modality, and classical theism.

2. Historical and Intellectual Context

2.1 The 13th‑Century University Setting

De Ente et Essentia emerged in the milieu of the University of Paris in the mid‑13th century, when the arts and theology faculties were assimilating a vast corpus of newly translated Aristotelian and Arabic philosophical texts. The Parisian curriculum emphasized:

Within this framework, there was a need for concise treatises that clarified technical notions—such as being, essence, and cause—at the intersection of philosophy and theology.

2.2 The Aristotelian and Avicennian Background

Aquinas’s analysis presupposes the Aristotelian distinction between substance and accidents, the ten categories, and the act–potency scheme. At the same time, scholars widely agree that De Ente et Essentia is deeply shaped by Avicenna’s doctrine of essence and existence:

• Avicenna’s essence–existence distinction and his claim that essences are “neutral” with respect to existing influenced Aquinas’s starting points.
• The Avicennian idea that existence is something “added” to essence in contingents provided a template that Aquinas reworked into his own account of esse as act.

Some historians emphasize the role of Augustinian and Neoplatonic traditions (mediated through Pseudo‑Dionysius and Proclus), especially in Aquinas’s talk of participation and emanation-like causality, although these influences are more implicit here than in his theological works.

2.3 Scholastic Debates and Institutional Constraints

When Aquinas wrote, controversies surrounded:

• the legitimacy of Aristotle’s natural philosophy and metaphysics within Christian theology;
• the interpretation of Avicenna and Averroes on the eternity of the world, the unity of the intellect, and the status of universals.

In this context, De Ente et Essentia can be read as part of an effort to articulate a Christian Aristotelian metaphysics that could coexist with doctrinal commitments such as creation ex nihilo and divine simplicity. The text does not directly engage ecclesiastical condemnations, but later interpreters sometimes relate its positions to broader attempts to regulate Aristotelian teaching (e.g., the Paris condemnations of 1270 and 1277).

3. Author and Composition of De Ente et Essentia

3.1 Aquinas’s Early Career

Thomas Aquinas (1224/5–1274), a Dominican friar, studied first at Naples, then in Paris and Cologne. Under Albert the Great, he absorbed the new Aristotelian corpus and commentarial tradition. De Ente et Essentia is generally dated to his early scholarly period, around 1252–1256, either during his first Parisian regency as master in theology or shortly before it, possibly in Italy.

3.2 Purpose and Intended Audience

Most scholars regard the treatise as written for advanced students in the arts or theology faculties who needed clarification of difficult metaphysical terminology. The work’s style—compact, technical, and almost textbook-like—suggests a didactic purpose:

• It presupposes training in logic and basic metaphysics.
• It focuses on conceptual clarification and orderly exposition rather than on scriptural exegesis.
• It rarely appeals to specifically Christian doctrines, supporting the view that it was composed within, or at least for, a philosophical rather than a narrowly theological context.

Some interpreters see it as a propaedeutic to Aquinas’s later commentary on Aristotle’s Metaphysics and to his theological synthesis in the Summa Theologiae.

3.3 Date and Circumstances of Composition

The exact date is debated. Two main hypotheses are commonly discussed:

There is no explicit dedication in the text itself, but a traditional view locates its composition within the environment of Dominican study houses connected to Paris.

3.4 Place in Aquinas’s Corpus

Later Thomistic commentators tend to treat De Ente et Essentia as programmatic for Aquinas’s metaphysics, especially regarding:

• the essence–existence distinction;
• the characterization of God as ipsum esse subsistens.

More recent historians adopt a developmental approach, noting differences between this treatise and later works, and suggesting that Aquinas’s views on some issues—such as the status of separate substances or the analysis of analogy—may have evolved. Nonetheless, the text is widely regarded as an early, concentrated statement of themes that remain central throughout his career.

4. Structure and Organization of the Treatise

De Ente et Essentia is divided into six relatively brief chapters, each building on the preceding one in a tightly structured progression from semantic analysis to metaphysical conclusions.

4.1 Overview of the Six Chapters

4.2 Logical Progression

• Chapter 1 begins with the term “being” (ens), distinguishing its uses (as divided by the categories and as the truth of propositions) and then introduces essence as what is signified by a definition.
• Chapter 2 narrows the focus to corporeal, composite substances, analyzing how prime matter and substantial form combine to yield a definable nature.
• Chapter 3 extends the inquiry to immaterial beings (such as separate intellects), while also explaining how one and the same essence can be considered as universal (in the mind), particular (in individuals), and as it exists in reality.

Building on this:

• Chapter 4 articulates the metaphysical relation between essence and existence in created beings, arguing for their real distinction.
• Chapter 5 then places this distinction within a more general survey of different kinds of ontological composition, showing that all creatures, even immaterial ones, are in some way composite.
• Chapter 6 finally infers the existence of a first being in which no composition occurs, especially not that of essence and existence.

4.3 Internal Method and Style

Within chapters, Aquinas typically adopts a pattern:

1. Identify a terminological or conceptual difficulty.
2. Offer distinctions (e.g., multiple senses of “being” or “essence”).
3. Use examples (e.g., human, animal) to illustrate genus–difference structure.
4. Draw doctrinal conclusions that prepare the way for later steps.

There is little explicit polemic; objections and alternative views are usually implied rather than directly quoted. This contributes to the treatise’s reputation as a clear, methodical introduction to metaphysical issues, rather than as a commentary or disputation.

5. The Concept of Being (Ens) in Aquinas

5.1 Two Principal Senses of “Being”

In De Ente et Essentia Aquinas begins by distinguishing two main senses of being (ens):

1. Being as divided by the categories: that which falls under the ten Aristotelian categories (substance and nine types of accidents). This is the primary sense relevant to metaphysics.
2. Being as the truth of propositions: whatever can be affirmed or denied in a true statement (e.g., “blindness is in the eye”). Here “being” signifies not an entity in the world, but the truth-value of assertions.

Aquinas maintains that metaphysics focuses chiefly on the first sense, while presupposing the logical sense as part of its linguistic apparatus.

5.2 Being and Substance

Within categorial being, substance occupies a privileged position. Substances exist in themselves, while accidents exist in substances. Aquinas’s treatment presupposes this hierarchy:

• Substances are primary instances of being.
• Accidents are secondary and dependent modes of being.

This framework underlies his later analysis of essence as first and foremost the essence of substances, with the essences of accidents considered by analogy.

5.3 Being and the Analogy of Predication

While De Ente et Essentia does not offer a full doctrine of the analogy of being, it contains elements that later Thomists cite in support of analogical predication:

• “Being” is not said univocally of all things, since it applies both to substance and accident, and eventually to God and creatures, in different ways.
• Nor is it purely equivocal, since there is an ordered similarity grounded in the act of existing (esse).

Commentators disagree on how fully developed Aquinas’s analogical theory is in this text. Some argue that the seeds of his later account are already present; others see the treatise as primarily concerned with categorial being rather than with analogy between God and creatures.

5.4 Being and Act (Esse as Act)

A crucial feature of Aquinas’s concept of being is the identification of existence (esse) as an act that actualizes an essence. In this treatise, “being” can refer:

• sometimes to the composite (the existing thing as a whole),
• sometimes to existence as the act whereby an essence is.

This dual use sets the stage for his distinction between essence (what a thing is) and esse (that by which it is), developed more explicitly in later chapters.

6. Essence, Quiddity, and Definition

6.1 Essence as What Is Signified by the Definition

Aquinas defines essence (essentia) or quiddity (quidditas) as “that which is signified by the definition of a thing.” It answers the question “What is it?” and captures the nature or kind to which individuals belong. In De Ente et Essentia:

• Essence is presented as a metaphysical correlate of a correct definition.
• The study of essence therefore presupposes logical analysis of genus and difference.

6.2 Quiddity and Nature

Though used largely interchangeably, Aquinas sometimes nuances these terms:

The treatise tends to focus on quiddity and essence as definable, leaving detailed discussion of nature-as-tendency for other works.

6.3 Genus and Difference

Aquinas uses the Aristotelian framework of genus and differentia to analyze definitional structure:

• Genus indicates what is common to many species (e.g., “animal”).
• Difference indicates what sets one species apart within a genus (e.g., “rational”).

In material species, the genus is connected with matter considered under a common aspect, while the specific difference is tied to form. For example, in “rational animal,” animal corresponds to a more general material nature, and rational to the form that specifies the human species.

6.4 Essence as Universal, Particular, and in the Mind

The treatise distinguishes different “modes” in which essence is considered:

1. As it exists in individuals (e.g., this particular human), where it is neither universal nor multiplied as an abstract object.
2. As universal in the mind, where the intellect abstracts from individuating conditions and grasps the common nature.
3. As in itself, prescinding from whether it is instantiated or conceived, where essence is said to be “indifferent” to the one and the many.

This tripartite analysis underpins Aquinas’s moderate realism about universals: essences are neither mere names nor separate Platonic forms, but have a basis in individual things and are universal in the intellect.

7. Essence in Material and Immaterial Substances

7.1 Material Substances: Matter and Form

For material substances, Aquinas holds that essence is constituted by the composite of prime matter and substantial form. Prime matter is pure potentiality, never found apart from form; substantial form actualizes matter, giving rise to a determinate species.

Key points in De Ente et Essentia:

• The essence of a corporeal substance includes both matter and form, not form alone.
• Nevertheless, definitions typically mention matter only under a common aspect (e.g., “body,” “animal”), not as individuated matter.
• Accidents (such as quantity or color) modify the substance but do not belong to its essence; they are excluded from strict definitions.

7.2 The Role of Matter in Individuation

Aquinas connects individuation in material substances to matter designated by quantity (“this flesh and these bones”). While the treatise does not give a full theory of individuation, it indicates that:

• The same specific essence (e.g., humanity) is multiplied in different individuals through distinct parcels of matter.
• Essence as such is common, but not as a separate universal; rather, it is instantiated in many via matter.

This prepares for his later account of how universals relate to individual beings.

7.3 Immaterial Substances: Separate Forms and Intellects

Chapter 3 turns to immaterial substances (e.g., angels, separate intellects). Here:

• There is no composition of prime matter and form.
• The essence is identified primarily with form itself, which exists without being the act of any underlying matter.

Aquinas nonetheless maintains that immaterial substances are not identical with their esse (act of existence); they still have a composition of essence and existence, discussed more fully in later chapters.

7.4 Degrees of Simplicity

The treatise presents a gradation:

Immaterial creatures have essences that are simpler than those of material beings (no matter–form), but they remain finite and composed in relation to their act of existence. This graded account of simplicity situates both material and immaterial essences within a common metaphysical framework.

8. The Real Distinction between Essence and Existence

8.1 Formulation of the Distinction

In chapter 4, Aquinas argues that in all created beings, essence (what a thing is) is really distinct from existence (esse: that by which it is). The distinction is not merely conceptual or linguistic; it corresponds, according to his analysis, to an ontological composition within things.

He supports this by noting that:

• An essence can be understood without knowing whether it is instantiated (e.g., one can understand “phoenix” without knowing that any exist).
• Essences are in principle common to many individuals, whereas existence is the act that makes a given essence actually present in reality.

8.2 Essence as Potency, Existence as Act

Within the broader framework of act and potency, Aquinas characterizes the relation thus:

• Essence is potential with respect to existence; it is a capacity or aptitude to be.
• Existence is act, the perfection by which an essence is made to be.

In De Ente et Essentia, this is presented as a metaphysical structure analogous to, but more fundamental than, matter–form composition: even where there is no matter, there remains an essence that can either be or not be, and an act of being that actualizes it.

8.3 Arguments Employed

The treatise offers several lines of reasoning, including:

• From multiplicity of beings of the same species: If essence and existence were identical in a finite being, that being would be necessary and unique in its kind. Since many individuals share one species (e.g., many humans), their existence cannot belong to the essence itself.
• From contingency and causality: Created beings are contingent; they can either exist or not. This suggests that their existence is received from another, and what receives must be distinct from what is received.

Scholars debate the logical force of these arguments. Some see them as implicitly relying on Avicennian premises about contingency; others interpret them as grounded in Aquinas’s own account of causality and participation.

8.4 Alternative Interpretations

Subsequent thinkers have proposed different readings of the distinction:

Debates about which reading best fits De Ente et Essentia continue, with some scholars emphasizing textual evidence for a robust real distinction, and others highlighting ambiguities in Aquinas’s language and the role of logical analysis.

9. Act, Potency, and Participation

9.1 Act and Potency as a General Framework

Throughout De Ente et Essentia, Aquinas presupposes the Aristotelian pair act (actus) and potency (potentia) as a universal explanatory scheme:

• Potency is capacity or ordered possibility for a given perfection.
• Act is the realized state that fulfills this capacity.

This scheme underlies:

• matter–form composition in material substances (matter as potency, form as act),
• the relation of essence to existence (essence as potency, esse as act).

9.2 Essence–Existence within Act–Potency

In the treatise, the most original application of act–potency concerns the relation between essentia and esse:

• Every finite essence is potentially many; existence limits it to a particular, concrete instantiation.
• Existence is said to be “received” in essence; what receives is in potency, and what is received is in act.

This move allows Aquinas to extend an originally physical framework (act/potency in change and motion) into a more strictly metaphysical register, where “act” means primarily to be rather than merely “to move” or “to operate.”

9.3 Participation in Being

Although De Ente et Essentia does not elaborate a full theory of participation, it employs participatory language to describe how creatures have being:

• Created things do not have esse essentially; they receive it.
• Their being is therefore a participation in a more fundamental act of being.

Participation here implies:

• that finite beings possess a perfection (being) in a limited, derivative way;
• that there is, at least in principle, a source in which that perfection is found without limitation.

Later Thomistic interpreters link this participatory structure directly to the treatise’s final claim about God as ipsum esse subsistens, while some historians caution that the text itself remains relatively terse on the mechanics of participation.

9.4 Hierarchy of Acts

Implicit in the treatise is a hierarchy:

This ordering allows Aquinas to treat esse as the most fundamental “act,” a point that will be central for later developments in Thomistic metaphysics.

10. God as Ipsum Esse Subsistens

10.1 From Essence–Existence Composition to a First Being

In the final chapter, Aquinas argues that since all created beings exhibit a composition of essence and existence, there must be a first cause in which this composition does not obtain. The reasoning is broadly causal:

• Whatever has existence as received participates in it and depends on another.
• A regress of such dependent beings, each receiving existence, calls for a source that does not receive existence but is existence.

This source is identified with God.

10.2 God’s Essence as Existence

Aquinas characterizes God as ipsum esse subsistens—“subsistent being itself.” In this conception:

• God’s essence is not distinct from his existence; God’s nature is to be.
• God is not a member of any genus, even that of “being,” because genus implies a composition of common nature and individuating difference, which would reintroduce composition.

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“Therefore it must be that there is a being whose essence is its existence.”
— Thomas Aquinas, De Ente et Essentia, ch. 6 (paraphrased across translations)

10.3 Divine Simplicity

From the identification of essence and existence in God, Aquinas infers divine simplicity:

• God lacks all metaphysical composition—no matter–form, no substance–accident, no essence–existence.
• Any composition would require a cause or factor uniting components, incompatible with the idea of God as the uncaused first being.

This simplicity differentiates God not only from material substances but also from created immaterial substances, which still possess an essence distinct from their act of being.

10.4 Interpretive Debates

Later commentators and critics offer differing assessments:

• Some Thomistic interpreters see De Ente et Essentia as offering a compressed but complete demonstration of God as pure act of being, foundational for later proofs.
• Others argue that the move from the composition in creatures to a unique simple being tacitly presupposes further metaphysical theses (about causality, participation, and the impossibility of infinite regress) not fully argued in the text.
• Debates continue over how this account of God’s identity with his own existence relates to issues such as divine freedom, knowledge of creatures, and personal attributes, though these themes lie largely outside the scope of the treatise itself.

11. Key Concepts and Technical Vocabulary

De Ente et Essentia relies on a dense network of technical terms. Some of the most central include:

11.1 Logical–Metaphysical Interface

Several terms straddle logic and metaphysics:

• Genus and differentia structure definitions; metaphysically, they often correspond to aspects of matter (under a common aspect) and form.
• Universal and particular designate modes of essence: universal in the intellect, particular in individuals.

11.2 Degrees of Composition

The treatise distinguishes multiple kinds of composition:

A clear grasp of these terms and their interrelations is essential for following Aquinas’s arguments, since he often moves from logical distinctions (e.g., between definition’s parts) to metaphysical ones (e.g., between real principles of being).

12. Philosophical Method and Use of Authorities

12.1 Method: From Linguistic Clarification to Metaphysical Analysis

Aquinas’s method in De Ente et Essentia proceeds from semantic and logical clarification to metaphysical claims:

1. Clarify terms: Analyze how words like “being” and “essence” are used (multiple senses, categorial vs. propositional).
2. Identify conceptual relations: Show connections between genus, difference, matter, and form.
3. Infer ontological structures: Move from distinctions in definition to distinctions in reality (e.g., essence–existence).

This reflects a characteristic medieval approach in which logical analysis is not purely formal but is treated as a guide to metaphysical structure.

12.2 Use of Aristotelian Frameworks

Aquinas systematically employs Aristotelian tools:

• The categories provide the initial division of being.
• The act–potency pair structures accounts of change, composition, and essence–existence.
• The genus–species–difference scheme underlies his treatment of definition.

He rarely cites Aristotle explicitly in this brief treatise, but the dependence on Aristotelian doctrines is widely recognized. Some historians note that Aquinas occasionally modifies Aristotle in light of later developments (e.g., extending act–potency beyond physical change).

12.3 Engagement with Avicenna and Other Authorities

While De Ente et Essentia mentions few names, its arguments resonate strongly with Avicenna’s Metaphysics:

• The distinction between essence and existence and the notion that essences are indifferent to existing recall Avicennian positions.
• The idea of a necessary being whose essence is existence itself parallels aspects of Avicenna’s proof of the Necessary Existent.

Aquinas, however, adapts these ideas within his own framework, especially in connection with act–potency and participation.

12.4 Role of Theological Authorities

Unlike Aquinas’s theological works, this treatise contains minimal explicit appeal to Scripture or Church Fathers. When theological implications arise (e.g., in the discussion of God as subsistent being), they are presented chiefly in philosophical terms. This has led many scholars to classify De Ente et Essentia as a primarily philosophical work that nonetheless prepares the ground for theological elaborations elsewhere.

12.5 Disputation and Commentary Context

The style of the treatise is more linear exposition than full scholastic disputation:

• Objections are usually implicit rather than formally stated.
• Authorities are appealed to mainly through conceptual frameworks rather than extended quotation.

This suggests that De Ente et Essentia may have functioned as a kind of introductory manual or prologue to more formal disputations and commentaries where authorities and objections would be handled in greater detail.

13. Famous Passages and Doctrinal Highlights

13.1 Essence as Signified by Definition

One frequently cited passage defines essence as that signified by a definition:

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Essence is that which is signified by the definition of a thing.

— Thomas Aquinas, De Ente et Essentia, ch. 1 (across standard translations)

This formulation has become a canonical reference for scholastic discussions of quidditas and the relationship between logic and metaphysics.

13.2 The Two Senses of Being

Another notable section distinguishes the two main senses of “being”:

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Being is said in two ways. In one way of that which is divided by the ten categories; in another way, of that which signifies the truth of propositions.

— De Ente et Essentia, ch. 1

This distinction underlies many later medieval treatments of ontology versus truth and is often invoked in discussions of Aquinas’s semantics.

13.3 Material Essence as Matter–Form Composite

In chapter 2, Aquinas’s account of material essence is a key point of reference:

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In things composed of matter and form, the essence includes both, although in definitions the form is more properly signified.

— De Ente et Essentia, ch. 2 (paraphrased)

This statement has been central to debates concerning whether essence in corporeal substances is primarily formal or truly a composite of matter and form.

13.4 The Real Distinction Thesis

The treatise’s articulation of the real distinction between essence and existence is among its most discussed doctrinal highlights:

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In created things, therefore, existence is something other than their essence.

— De Ente et Essentia, ch. 4 (paraphrased)

This brief formulation has generated extensive commentary, particularly in Thomistic and analytic metaphysics.

13.5 God as Subsistent Being Itself

Finally, chapter 6’s description of God has become emblematic of Aquinas’s metaphysics of God:

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It is necessary to posit something which is its own existence, and this is God.

— De Ente et Essentia, ch. 6 (paraphrased)

This passage encapsulates the move from finite beings, in which essence and existence are distinct, to a first being whose essence is its existence, a key doctrinal highlight for later discussions of divine simplicity and classical theism.

14. Reception, Debates, and Criticisms

14.1 Medieval Reception

Within late 13th‑ and 14th‑century scholasticism, De Ente et Essentia was widely circulated and commented upon, especially in Dominican circles. It contributed to Aquinas’s reputation as a leading interpreter of Aristotle and Avicenna.

Later Thomists, such as Cajetan, produced detailed line‑by‑line commentaries that treated the treatise as an authoritative summary of Thomistic metaphysics. At the same time, other schools reacted critically:

• Scotists often accepted some distinction between essence and existence but interpreted it as a formal rather than a fully real distinction, questioning Aquinas’s account of the composition within creatures.
• Nominalists tended to reduce many of Aquinas’s real distinctions (e.g., between essence and existence, between substance and accidents) to conceptual or linguistic differences, thereby challenging the treatise’s ontological commitments.

14.2 Early Modern and Modern Readings

In the early modern period, interest in the treatise declined outside scholastic circles, but it remained a staple in Catholic seminaries and Thomist schools. The 19th‑ and 20th‑century Thomistic revival brought renewed attention, with scholars such as Étienne Gilson and Joseph Owens emphasizing its centrality for understanding Aquinas’s metaphysics of esse.

Critics from analytic and continental traditions have raised several concerns:

• Some question the coherence of the essence–existence distinction, arguing that “existence” is not a predicate or real component, but at best a logical or quantificational notion.
• Others challenge the inference from the composition in creatures to the existence of a unique simple being, viewing it as dependent on contested assumptions about causality and participation.
• Concerns about divine simplicity—that identifying God with his existence renders God too abstract or impersonal—are frequently directed at the conception articulated in the final chapter.

14.3 Intra‑Thomistic Debates

Even among Thomists, significant debates persist:

These debates show that the treatise functions not only as a historical document but also as a living point of reference for ongoing metaphysical and theological discussions.

15. Legacy and Historical Significance

15.1 Influence on Later Scholasticism

De Ente et Essentia played a significant role in shaping later medieval and early modern scholastic metaphysics. It became:

• a standard introductory text in Dominican and other Catholic schools;
• a primary source for Thomistic doctrines of essence–existence, participation, and divine simplicity.

Commentators such as Cajetan, John of St. Thomas, and later neo‑Thomists elaborated complex systems around its key claims, often treating it as the most concise statement of Aquinas’s metaphysical outlook.

15.2 Place in the History of Metaphysics

Historians often situate the treatise at a crossroads of Aristotelian, Avicennian, and Augustinian/Neoplatonic traditions:

• It transmits and transforms Avicenna’s distinction between essence and existence.
• It extends Aristotelian act–potency theory into a more explicitly ontological register focused on existence as act.
• It offers a philosophically articulated vision of God as pure being, which would later inform classical theistic conceptions.

In surveys of medieval philosophy, De Ente et Essentia is frequently cited as a pivotal text for understanding the Latin West’s appropriation of Arabic philosophy and the development of scholastic ontologies.

15.3 Modern Philosophical Engagements

In the 20th and 21st centuries, the treatise has been engaged from multiple angles:

• Existential Thomists (e.g., Gilson) highlight its emphasis on esse as central to Aquinas’s originality, contrasting it with “essentialist” metaphysics.
• Analytic philosophers interested in metaphysical grounding, ontological dependence, and modal structure draw on the essence–existence framework as an historically rich alternative to contemporary models.
• Discussions of classical theism and divine simplicity frequently reference its final chapter as a concise philosophical articulation of God as ipsum esse subsistens.

15.4 Ongoing Relevance

The treatise continues to be taught and discussed both in historical courses on medieval philosophy and in systematic metaphysics and theology. Its enduring significance lies in:

• the clarity with which it connects logical analysis, metaphysical structure, and theology;
• its influential articulation of how being, essence, and causality interrelate;
• its role as a touchstone in debates over the viability of classical metaphysical frameworks in contemporary thought.

While interpretations of its central theses diverge, On Being and Essence remains a key reference for anyone investigating medieval ontologies or the broader history of metaphysical inquiry into what it is to be.