In metaphysics and formal ontology, the distinction between what an entity fundamentally is and what it merely happens to be represents one of the oldest and most consequential inquiries in Western philosophy. An accidental property characterizes an object without constituting its core identity or determining its persistence through change. Understanding the mechanics of accidental properties illuminates how objects navigate time, variation, and counterfactual possibilities while remaining numerically identical to themselves.
Accidental Property
1. Concise Definition
An accidental property (Latin: accidens; Greek: symbebēkos) is a characteristic, attribute, or feature that an entity possesses contingently rather than necessarily. In standard modal metaphysics, a property $P$ is accidental to an object $x$ if and only if $x$ possesses $P$, but there exists at least one possible world in which $x$ exists without possessing $P$. Consequently, an object can acquire, lose, or vary in its accidental properties without ceasing to exist or transforming into a radically different kind of entity.
In contrast to an essential property—which an entity must instantiate across all possible worlds where it exists in order to maintain its ontological identity—an accidental property is metaphysically non-constitutive. For example, while having three interior angles summing to 180 degrees is an essential property of a Euclidean triangle, being drawn in red ink on a chalkboard is an accidental property of that particular triangle. The removal or modification of an accidental property alters how the object is qualified, but it leaves the fundamental nature and identity conditions of the substance completely intact.
2. Etymology & Linguistic Origin
The philosophical taxonomy of the accidental property originates in the classical Greek tradition. Aristotle introduced the technical construct in his logical and metaphysical treatises using the term symbebēkos (συμβεβηκός), the neuter perfect participle of the verb symbainein (συμβαίνειν), meaning “to happen together,” “to coincide,” or “to fall out concurrently.” Within the Aristotelian corpus, the term designated that which belongs to something and can be truly asserted of it, but neither belongs of necessity nor constitutes the definition of the subject’s essence (to ti ēn einai).
During the Roman and medieval eras, Hellenic philosophical concepts were translated systematically into Latin. Roman philosophers, notably Boethius and Cicero, rendered symbebēkos as accidens, derived from the Latin verb accidere (“to fall upon,” “to happen,” or “to occur by chance”), formed from the prefix ad- (“to” or “upon”) and cadere (“to fall”). Medieval Scholastics, most prominently Thomas Aquinas and John Duns Scotus, formalized the distinction between substantia (substance) and accidentia (accidents). The term entered Middle English during the late fourteenth century via Old French and Scholastic Latin, establishing the English noun phrase “accidental property” within post-Renaissance metaphysics and natural philosophy.
3. Pronunciation & Grammatical Form
Pronunciation: Phonetically transcribed in the International Phonetic Alphabet (IPA) as /ˌæk.sɪˈdɛn.təl ˈprɒp.ər.ti/ (British English) or /ˌæk.səˈdɛn.təl ˈprɑː.pɚ.t̬i/ (American English).
Grammatical Form: Compound noun phrase comprising the descriptive adjective “accidental” (acting as a classifying modifier) and the abstract mass/count noun “property.” The plural form is accidental properties. Philosophically, it functions as a second-order predicate in monadic or polyadic logic, designating attributes ascribed contingently to a primary substance or entity ($x$). The adverbial form is accidentally (e.g., “the object is accidentally colored”), and the related substantive noun is accident (used classically to mean a contingent property rather than an unintended mishap).
4. Detailed Conceptual Explanation
To grasp the conceptual scope of an accidental property, one must examine the broader metaphysical framework of substance-attribute ontology. In traditional substance theories, the world is populated by primary entities called substances (such as an individual human, an oak tree, or an atom of gold) that serve as independent ontological anchors capable of bearing attributes. These attributes are bifurcated into two mutually exclusive and exhaustive categories: essential properties and accidental properties. An essential property determines the object’s ontological genus and persistence conditions; if an object loses an essential property, it undergoes substantial change—meaning it ceases to exist entirely or is annihilated. By contrast, when an object gains or sheds an accidental property, it undergoes mere accidental change, persisting through time with its core identity uninterrupted.
In contemporary analytic philosophy, this ancient distinction is formalized within modal logic and possible worlds semantics. Under the standard Kripkean modal framework, let $W$ denote the set of all possible worlds, and let $E(x, w)$ denote that object $x$ exists in world $w$. A property $P$ is an accidental property of $x$ in the actual world $w_@$ if and only if: (1) $x$ possesses $P$ in $w_@$; (2) $E(x, w_@)$; and (3) there exists at least one possible world $w_i in W$ such that $E(x, w_i)$ and $
eg P(x, w_i)$. Under this formulation, contingency is the defining condition of the accidental. Accidental properties permit counterfactual assertions: one can say that “Socrates could have been a musician rather than a philosopher” without asserting that Socrates would have ceased to be the historical person Socrates.
The conceptual boundary of accidental properties also interfaces with the distinction between intrinsic and extrinsic (relational) features. An accidental property can be intrinsic to the subject—such as the surface temperature of an iron kettle or the specific mass of a growing puppy—operating within the physical boundaries of the entity without determining its core identity. Alternatively, accidental properties are frequently extrinsic or relational, defined by an entity’s contingent connections to external entities. For example, Socrates being taller than Theaetetus or residing in Athens are extrinsic accidental properties; their alteration requires no internal physical reordering of Socrates himself, yet both predicates accurately describe his contingent state in the actual world.
Furthermore, philosophers distinguish between qualitative accidental properties and quantitative accidental properties. Qualitative accidents involve sensible or phenomenological modifications, such as an apple transitioning from green to red as it ripens. Quantitative accidents involve measurable extensions, such as an organism gaining weight or a river swelling after rainfall. In every instance, the accidental status of the property is circumscribed by identity criteria: as long as the change does not violate the sortal concept under which the entity falls, the altered property remains strictly accidental.
5. Historical Development
The philosophical investigation of accidental properties originated in antiquity with Aristotle’s rejection of Platonic realism. In works such as the Categories, Topics, and Metaphysics, Aristotle sought to explain how physical objects could endure through real material change without losing their underlying reality. Plato had largely relegated the shifting physical realm to the status of imperfect becoming, arguing that true being resided only in the immutable Forms. Aristotle resolved this by positing the primary substance (ousia) as the foundational subject of predication, arguing that accidents (symbebēkota) inhere in substances. For Aristotle, an accident is that which belongs to a subject without being part of its definition, occurring neither “always nor for the most part” as an absolute necessity of nature.
During late antiquity, the Neoplatonist philosopher Porphyry refined the Aristotelian categories in his highly influential treatise Isagoge. Porphyry formulated the “five predicables”—genus, species, difference, proprium (peculiar property), and accident. Porphyry introduced a crucial refinement between separable accidents (which can leave the subject in physical reality, such as a human sleeping or standing) and inseparable accidents (which cannot be physically separated from the subject in actuality, such as the blackness of a crow, even though they remain conceptually distinct from the species’ core essence). This text became the primary pedagogical vehicle through which early medieval thinkers encountered categorical logic.
In the high medieval period, the nature of accidental properties became a matter of acute theological and metaphysical debate. Scholastic philosophers including Thomas Aquinas, John Duns Scotus, and William of Ockham examined the ontology of accidents, spurred in part by the doctrine of transubstantiation in the Catholic Eucharist. Aquinas argued that during the Eucharistic consecration, the underlying substance of bread and wine is replaced by the body and blood of Christ, while the accidents (color, taste, texture, spatial extension) persist miraculously through divine power without their natural material substratum. This intense theological scrutiny forced Scholastics to analyze the metaphysical status of accidents with extreme rigor, debating whether accidents possessed their own secondary degree of being (esse accidentale).
The Scientific Revolution and the dawn of modern philosophy marked a sharp break with Aristotelian-Scholastic taxonomy. Thinkers such as René Descartes, John Locke, and Thomas Hobbes dismantled the Scholastic machinery of substantial forms and real accidents. Locke reconceptualized properties through the lens of primary and secondary qualities; primary qualities (solidity, extension, figure, motion) were seen as real, inseparable properties of matter, while secondary qualities (color, taste, sound) were understood as powers in objects to produce sensations in observers. David Hume radicalized this empiricist trajectory by rejecting the existence of any underlying “substance” altogether, presenting the “bundle theory” wherein an object is nothing more than a collection of co-occurring, contingent perceptions with no metaphysical core beneath them.
In the twentieth century, the accidental-essential distinction underwent a major revival through modal metaphysics. Willard Van Orman Quine had fiercely attacked the distinction in the 1950s, arguing in From a Logical Point of View that essentialism was an incoherent metaphysical holdover that produced fatal paradoxes when combined with quantified modal logic (illustrated by his famous example of the “mathematical cyclist”). However, Saul Kripke revitalized the distinction in his landmark 1972 lectures Naming and Necessity. Kripke demonstrated that necessity and apriority are not coextensive, establishing that an object has its identity across possible worlds fixed by rigid designators, thereby formalizing the boundary between counterfactually contingent (accidental) and necessary (essential) properties.
6. Theoretical Foundations
The concept of an accidental property rests upon several interconnected frameworks within contemporary ontology and philosophical logic. The foremost framework is possible worlds semantics, pioneered by Kripke, David Lewis, and Jaakko Hintikka. Within this modal framework, properties are construed as functions from possible worlds to sets of individuals. An accidental property of an object $x$ is one whose modal extension includes $x$ in the actual world but excludes $x$ in some non-actual possible worlds in which $x$ nonetheless exists. This modal model formalizes the intuition that reality contains alternative ways things could have been without altering what things fundamentally are.
A competing contemporary framework is the non-modal or definitional account of essence and accident, formulated by Kit Fine. In his seminal 1994 paper “Essence and Modality,” Fine demonstrated that modal criteria fail to capture our pre-theoretical understanding of essence. Fine pointed out that while it is metaphysically necessary that Socrates belongs to the singleton set {Socrates} if he exists, containing Socrates is not part of what Socrates essentially is. Fine argued that essence precedes modality: an essential property is one that enters into the real definition of what an entity is, whereas an accidental property is any property possessed by an entity that lies outside that defining essence, regardless of its modal behavior across counterfactual states.
A third foundational pillar concerns theories of individuation and substance. Two major rival architectures govern the relationship between objects and their accidental properties:
- Substratum Theory: Posits that an object consists of an underlying subject or “bare substratum” that instantiates or supports both essential and accidental properties. The substratum provides numerical identity, allowing accidental properties to adhere, alter, or detach across time without threatening the underlying existence of the entity.
- Bundle Theory: Denies the existence of any underlying substratum, holding that concrete objects are nothing over and above bundles of co-instantiated properties. For bundle theorists, explaining accidental change is notoriously challenging, often requiring the adoption of counterpart theory or four-dimensionalism (perdurantism), where an object is extended through time and its accidental properties vary across different temporal parts or stages.
7. Key Components, Types & Dimensions
Accidental properties encompass several distinct dimensions and subcategories based on their modal behavior, physical manifestation, and relationship to the subject:
- Separable Accidents: Properties that an object can lose in the actual world at some point during its temporal lifespan while continuing to exist. Example: A human being’s posture (e.g., sitting versus standing) or the color of a painted wooden chair.
- Inseparable Accidents: Properties that an object cannot shed in physical reality during its actual existence, but which remain counterfactually contingent because their absence does not contradict the object’s ontological definition. Example: The exact geographic birthplace of an individual or the natural pigmentation of an animal’s coat.
- Intrinsic Accidental Properties: Contingent attributes that depend solely on the internal constitution and physical makeup of the object itself, independent of other external entities. Example: The core temperature of a stone or the internal pressure of an inflated tire.
- Extrinsic (Relational) Accidental Properties: Contingent attributes that depend on the object’s external spatial, temporal, social, or causal relations to other entities. Example: Being three meters away from an open doorway or being an employee of a specific organization.
- Temporary vs. Permanent Accidents: Temporary accidents fluctuate across the timeline of an object’s life cycle (e.g., childhood dentition), whereas permanent accidents persist unbroken throughout its existence despite remaining non-essential (e.g., a tree bearing a scar caused by lightning during its initial year of growth).
- Dispositional Accidental Properties: Contingent capacities or tendencies to react under specific environmental triggers that an entity possesses without those dispositions defining its core kind. Example: An ordinary piece of glass being accidentally treated with a film that alters its shatter-resistance threshold.
8. Examples & Illustrative Cases
To crystallize how accidental properties operate across diverse ontological categories, consider the following concrete cases drawn from natural, artifactual, and institutional domains:
Case 1: The Bronze Statue
Consider a statue of Athena cast in solid bronze. The property of being composed of bronze and having spatial extension is widely viewed as essential to that physical piece of metal. However, the property of being painted olive green, being housed inside a climate-controlled museum in Paris, or possessing a minor surface scratch on the left forearm are accidental properties. If the conservator strips the paint, transfers the statue to a museum in Athens, or buffs out the scratch, the statue does not cease to be the statue of Athena. Its core identity conditions remain fully satisfied despite the loss or variation of these contingent characteristics.
Case 2: The Biological Organism (Homo sapiens)
Take the historical philosopher Socrates. According to standard essentialist frameworks, Socrates is essentially a rational animal, a biological human being, and the offspring of his specific biological parents (origin essentialism). In contrast, his accidental properties are legion: his snub-nosed appearance, his skill at dialectical interrogation, his residence in the demes of Alopeke, his marital status to Xanthippe, and his execution by hemlock poisoning. Had Socrates chosen a career as a stonecutter and lived quietly into old age without ever practicing philosophy, the historical individual Socrates would still have existed, demonstrating that his vocation and philosophical fame were purely accidental to his personhood.
Case 3: Artifactual Modification and Ship of Theseus Dynamics
Consider an ocean-going commercial cargo vessel. The vessel is constructed with a hull painted deep navy blue and fitted with an electronic navigation console manufactured by a specific electronics firm. The navy blue hull color and the specific manufacturer of the console are accidental properties. If the shipping conglomerate dry-docks the vessel, repaints the hull industrial orange, and retrofits the cockpit with a competing navigation system, the vessel retains its numerical identity and registration. The changes occur entirely at the level of accidental properties, illustrating how artifacts endure significant technical reconfiguration without undergoing ontological dissolution.
9. Measurement & Assessment
Because the boundary between accidental and essential properties is fundamentally metaphysical rather than strictly physical, assessing whether a given property is accidental requires rigorous logical, semantic, and modal diagnostics rather than sensory observation alone. Philosophers and logicians employ several systematic evaluative tests:
- The Counterfactual Identity Test: Evaluates whether an object $x$ can be coherently conceived to exist in an alternative possible world without property $P$. If one can construct a non-contradictory counterfactual scenario where $x$ lacks $P$ yet preserves its identity and sortal persistence conditions, $P$ is diagnosed as accidental.
- The Diachronic Persistence Criterion: Observes whether the object $x$ survives the physical removal, diminution, or alteration of $P$ across temporal duration in the actual world. If $x$ continues to instantiate its primary kind-concept after losing $P$ (e.g., an apple remaining an apple after turning brown), $P$ is empirically confirmed to be accidental.
- The Definitional / Real Definition Criterion (Finean Test): Formulates the rigorous real definition of the entity’s essence (its quiddity). Any true attribute that is not explicitly contained within or logically derived from this foundational definition is classified as an accident.
- Rigid Designation Analysis: Uses formal modal semantics to determine whether a designator picks out the same entity across all possible worlds where it exists without requiring the satisfaction of descriptive predicates associated with $P$. If the description involving $P$ can fail without the designation failing, the predicate designates an accidental feature.
10. Applications & Practical Significance
The philosophical taxonomy of accidental properties extends far beyond abstract metaphysical musings, exerting practical influence across several contemporary domains:
Law and Jurisprudence: Legal systems continuously distinguish between the essential identity of legal persons (or corporate bodies) and their accidental features. A corporation retains its contractual obligations, legal liabilities, and institutional identity even if its board of directors changes, its headquarters relocates, or its branding is overhauled. Similarly, in criminal jurisprudence, establishing the identity of a defendant relies on demonstrating that alterations in appearance (e.g., hair color, facial scars, weight shifts) are merely accidental changes that leave the culpable individual identical to the perpetrator.
Computer Science and Ontology Engineering: In software engineering, object-oriented programming (OOP), and knowledge representation (such as the Web Ontology Language, or OWL), systems architects build ontological schemas that differentiate between identity-defining attributes (primary keys, invariant class types) and mutable state properties (instance variables, dynamic attributes). A database modeling an employee class treats a person’s national identification number as immutable and identity-linked, while treating their current department, office number, and salary as accidental properties that can be updated continuously via transactions without instantiating a new entity.
Developmental and Cognitive Psychology: Developmental psychologists study how human children acquire sortal concepts and practice what is termed “psychological essentialism.” Research reveals that young children naturally distinguish between surface-level, accidental features (such as putting a costume on a dog) and essential identity (recognizing that the dog does not become a tiger merely by wearing stripes). Mapping how the human mind categorizes the external world depends directly on understanding our cognitive hardwiring regarding accidental versus essential properties.
11. Research & Empirical Evidence
While the categorization of properties is fundamentally a conceptual and metaphysical endeavor, significant empirical research within cognitive science, linguistics, and experimental philosophy examines how human cognition navigates the accidental-essential divide.
Pioneering cognitive research by Susan Gelman and Frank Keil into psychological essentialism demonstrates that humans possess an early-emerging, robust cognitive bias to assume that living kinds (such as animals and plants) harbor an underlying, unobservable essence that determines their true identity. In Keil’s celebrated transformation experiments (1989), children were presented with stories where an animal underwent surgical alterations—for instance, taking a raccoon, painting a white stripe down its back, and surgically implanting a pouch of smelly odor so that it outwardly matched a skunk. Older children systematically resisted the classification change, identifying the skunk-like properties as purely accidental modifications and asserting that the animal remained a raccoon. Conversely, when similar alterations were performed on artifacts (e.g., modifying a coffee pot to serve as a birdhouse), children were far more willing to accept that the artifact had changed kinds, indicating that human cognition applies the category of accidental properties differently to natural kinds than to artificial objects.
In experimental philosophy (X-Phi), researchers such as Shaun Nichols, Stephen Stich, and Edouard Machery have investigated cross-cultural semantic intuitions regarding Kripkean rigid designation and modal properties. Studies indicate that while there is notable cross-cultural agreement regarding the contingency of accidental physical traits (such as accidental bodily modifications or location), intuitions regarding the essentiality of social, moral, or psychological attributes exhibit intriguing variations across demographic groups, prompting ongoing research into whether the boundary between essential and accidental properties is universally intuitive or partially shaped by cultural linguistic habits.
12. Cultural & Cross-Cultural Considerations
The formal dichotomy between substance and accidental properties is historically tethered to the Indo-European linguistic structure and Western philosophical lineage, particularly its subject-predicate grammar. Examining this construct through comparative and cross-cultural philosophy reveals profound differences in how world traditions conceptualize identity and change:
Classical Indian Philosophy: In the orthodox Nyāya-Vaiśeṣika traditions, a robust substance ontology developed that closely mirrors the Aristotelian paradigm. The Vaiśeṣika system delineates seven foundational categories (padārthas), distinguishing sharply between substance (dravya), quality (guṇa), and action (karma). Qualities and actions inhere contingently in substances through the ontological relation of inherence (samavāya), functioning analogously to accidental properties. In radical contrast, Buddhist philosophy—specifically the Madhyamaka and Abhidharma schools—systematically dismantles the concept of substance through the doctrines of anatta (non-self) and pratītyasamutpāda (dependent origination). For Buddhist philosophers like Nāgārjuna, there is no underlying, permanent substance in which accidental properties can inhere; all phenomena are “empty” (śūnya) of inherent essence (svabhāva). Under this view, the conventional distinction between essential and accidental properties is a cognitive illusion generated by linguistic reification.
Classical Chinese Thought: Traditional Chinese philosophy, notably classical Daoism and Neo-Confucianism, emphasizes process ontology over substance ontology. In the Daoist framework of the Daodejing and Zhuangzi, the cosmos is characterized by continuous transformation (bian or hua) and the perpetual flow of qi (vital energy). Rather than conceptualizing an enduring static core bearing external “accidents,” entities are understood as dynamic, interdependent patterns of events and relations. The Western concern with determining whether a property is rigidly essential or contingently accidental is largely supplanted by an inquiry into how harmonious or disharmonious an entity’s current configuration is within the wider ecological and cosmic order.
13. Criticisms, Debates & Limitations
Despite its enduring presence in philosophical taxonomy, the construct of the accidental property remains a locus of vigorous debate and criticism:
- Quine’s Indeterminacy and Anti-Essentialist Critique: W.V.O. Quine argued that designating a property as accidental or essential depends entirely on how an object is described rather than on the object itself. In his famous paradox, Quine asked: Is a mathematical cyclist necessarily or accidentally rational? Considered as a human being, the person is essentially rational and accidentally two-wheeled; considered as a bicyclist, the person is essentially two-wheeled and accidentally rational. Quine maintained that properties cannot be separated into essential and accidental without smuggling in an illicit, unscientific dogmatism.
- The Threat of the Bare Substratum: Opponents of substance ontology argue that stripping away all accidental properties to locate the essential core leads to an absurdity: the “bare substratum” (an entity stripped of all qualities, which is empirically unobservable and conceptually unintelligible). Bundle theorists contend that once you discard this mysterious peg upon which accidents are hung, the distinction between substance and accident collapses into a simple set of co-present attributes.
- Social Constructivism and Nominalism: Critical theorists, feminists, and nominalists argue that classifying properties as essential versus accidental is frequently an exercise in normative or political power rather than objective discovery. In domains such as gender, race, and disability, classifications that were historically claimed to be natural, essential traits have been exposed as culturally contingent, social conventions. Nominalists argue that all property delineations are human linguistic impositions, rendering the accidental-essential boundary inherently conventional rather than natural.
- Kit Fine’s Modal Counterexamples: As noted, Kit Fine undermined the modal account of accidental properties by showing that standard modal definitions mistakenly classify certain necessary properties as essential. Because it is necessary that Socrates exists if and only if {Socrates} exists, modal logic must view belonging to {Socrates} as essential to Socrates, an outcome Fine dismissed as obviously wrong. This ongoing dispute highlights that defining exactly what makes a property “accidental” remains an open problem in modern metaphysics.
14. Related Terms & Distinctions
To avoid conceptual confusion, the accidental property must be systematically distinguished from several closely aligned philosophical terms:
- Essential Property: The exact antonym of an accidental property. An essential property is one that an object cannot lose without ceasing to exist or changing its fundamental kind across all possible worlds where it exists (e.g., gold having atomic number 79).
- Proprium (Peculiar Property): In Porphyrian and Scholastic logic, a proprium is a property that does not define the essence of a species, yet flows necessarily from that essence and belongs exclusively and universally to that species across its lifetime (e.g., the capacity to laugh in humans, or being capable of grammar). Unlike an accidental property, a proprium cannot be lost by the species, even though it is not the defining genus or difference.
- Haecceity (“Thisness”): The non-qualitative property of an entity that makes it this particular individual and not another (individuation). An accidental property is a qualitative or relational attribute that could in principle be shared by multiple objects, whereas haecceity is uniquely proper to a single individual.
- Contingent Property: Often used interchangeably with accidental property in informal speech. However, in rigorous modal semantics, “contingent” refers to the truth-value of a proposition across possible worlds, whereas “accidental” specifically describes the relationship between an entity and an attribute in predication.
- Intrinsic vs. Extrinsic Properties: Orthogonal distinctions. An accidental property can be either intrinsic (the surface temperature of a coin) or extrinsic (the location of the coin in a velvet pouch).
15. Summary / Key Takeaways
The concept of an accidental property serves as a cornerstone of logic, metaphysics, and ontology. It provides the analytical mechanism that explains how entities retain their enduring identity and structural coherence while undergoing real, observable changes across space and time. Whether an entity is a biological organism adapting to its environment, an artifact undergoing physical restoration, or an institutional body revising its corporate governance, the categorization of attributes into essential cores and accidental modifications preserves the intelligibility of identity through transformation.
Ultimately, the study of accidental properties reveals that our comprehension of reality is fundamentally split between what is invariant and what is contingent. By defining what an entity can safely surrender without vanishing into non-existence, the accidental property maps the boundaries of counterfactual possibility, charting the diverse alternate pathways an object could have traveled while remaining authentically itself.
References
- Aristotle. (1984). The complete works of Aristotle: The revised Oxford translation (J. Barnes, Ed.). Princeton University Press.
- Fine, K. (1994). Essence and modality. Philosophical Perspectives, 8, 1–16. https://doi.org/10.2307/2214160
- Gelman, S. A. (2003). The essential child: Origins of essentialism in everyday thought. Oxford University Press. https://doi.org/10.1093/acprof:oso/9780195154061.001.0001
- Keil, F. C. (1989). Concepts, kinds, and cognitive development. MIT Press.
- Kripke, S. A. (1980). Naming and necessity. Harvard University Press.
- Locke, J. (1975). An essay concerning human understanding (P. H. Nidditch, Ed.). Oxford University Press. (Original work published 1689).
- Porphyry. (2003). Isagoge (E. W. Warren, Trans.). Pontifical Institute of Mediaeval Studies.
- Quine, W. V. O. (1953). From a logical point of view: 9 logico-philosophical essays. Harvard University Press.