Introduction to Ontology

"Existence is prior to essence."--Jean-Paul Sartre

"Esse est percipi."--George Berkeley

"To be is to be the value of a bound variable."--Willard van Orman Quine

"It is only when the core of being has shown itself as social becoming, that the being itself can appear as a product, so far unconscious, of human activity, and this activity, in turn, as the decisive element of the transformation of being." --Georg Lukacs, Marxist philosopher

Alice C. Linsley


Ontology is the philosophical study of existence or the nature of being. Ontological questions include:

What is existence?
Is there a difference between existence and being?
Why does something exist?
What are the essential or incidental, as opposed to the accidental, traits of an entity?
Is existence a property?
If an entity changes in essence does it cease to be?
Are sensible substances the only substances that can be said to exist?
What is the relationship between consciousness and being?

Plato believed that entities are real because they have distinct essences patterned after the original immutable and eternal Forms. His Forms are immaterial and classified according to essence in a hierarchy. His student Aristotle was also an essentialist, but his classification of entities led him to a different view. This is taken up in Aristotle's writings, especially his Categories.


Aristotle’s approach to ontology

Aristotle attempted to categorize entities by four divisions: universal substances; universal non-substances; individual substances and individual non-substances. In Aristotle's writing "substance" refers to essence. He classifies essences as universal and particular, but he also recognizes non-essential universals and non-essential particulars. He uses the Greek phrase en tôi ti esti, en tôi ti ên einai (what belongs to a thing in respect of itself belongs to it in its essence). He defines essence/substance as "the what it is" (to ti esti). Essence or substance is one determinant of the entity's category. Another determinant is whether it represents a universal or a particular. Yet another determinant is what can be said about the entity.

Aristotle's categories are linked to the concept of boundaries or definition (horos, horismos). In his logical works, he says, “a definition is an account (logos) that signifies an essence” (Topics 102a3). He says that "what belongs to a thing in respect of itself belongs to it in its essence" (en tôi ti esti, en tôi ti ên einai) for we refer to it “in the account that states the essence” (Posterior Analytics, 73a34–5). The Greek expression to ti ên einai literally means "the what it was to be." Here we glimpse the necessity of teleos in Aristotle's reasoning. Essence for Aristotle is tied to the ultimate purpose or destiny of an entity, to its becoming what it is.

Aristotle is not the first to philosophically explore existence or being, but he is credited with laying the foundations of ontology. In Gamma 1 he asserts that humans desire to know and what they desire to know most are the "first principles and highest causes." He argues that these first principles are found by studying being qua being. By studying 'that which is' qua 'thing which is' and what belongs to 'that which is' we may discover first principles and highest causes. The 'qua' locution is used to indicate the general (propositional) principles that apply to 'that which is' and to consider their properties, or what Aristotle refers to as kath’ hauto (attributes). He further contends that there must be something to which the first principles and highest causes belong; they must be properties of being itself. He believes in “the Prime Mover” who is immutable, unmoved and complete in itself.


First Principles

In philosophy, a first principle is a basic, foundational proposition or assumption that cannot be deduced from any other proposition or assumption. For Aristotle a first principle is “the first basis from which a thing is known” (Metaphysics 1013a14–15).

To have knowledge (epistêmê) about birds, for example, is to be able to explain why birds are as they are and behave as they do. When we can give a fundamental explanation about birds as a class of being and explain the processes whereby birdness is achieved; we have come upon the first principles of birds. Aristotle seeks to establish what a bird is apart from but in relationship to what can be said about the individual bird (particular) and birds as a class (universal). He intends that this should be done by a well-considered process of observation and logic.


First Philosophy

“First Philosophy” is the term Aristotle uses in his Metaphysics for philosophy concerned with first causes and ultimate principles. He sees first causes as originating in the Nous. For Aristotle First Philosophy is at the root of the tree of knowledge. Aristotle maintains that causation is first and foremost the business of Philosophy. He uses the term archai to refer to the sources, origins or roots of things that exist and to the principles that provide the conditions of the possibility of things.

Aristotle's Four Causes (archai)

1. substance or essence
2. matter or substratum
3. source of change (efficient cause)
4. purpose and good (final cause)


Aristotle presents a logical view of Cause, Motion, and Purpose. He makes distinctions between kinesis (motion/change/flux/process) and energeia (power/actuality), and between actuality and potentiality. Here are some Aristotelian examples of actuality-potentiality pairs:


Potentiality                                            Actuality

stones/bricks                                          house
bronze                                                    statue
acorn                                                      oak tree


The Aristotelian distinction between kinesis and energeia is well presented in David Bradshaw’s book Aristotle East and West (Cambridge University Press, 2004), p. 10

Linguistic challenges

Aristotle's term "ousia" (essence) was rendered "substantia" in the Latin translations of his works. The English translations therefore use the word "substance" which connotes matter. Cicero recommended the Latin word "essentia" because Aristotle often speaks of non-material entities and universal essences.

Likewise, rendering "Horismos" or "Horos" as "definition" rather than "boundary" contributes to confusion. Horus was regarded as the one who establishes boundaries. The ancient Greeks were much enamored of Nilotic metaphysics and Plato studied under the Egyptian priest Sechnuphis wh would have taught him about the eternal soul and earthly reflections of heavenly realities. The notion of earth as a mirror of heaven is central to ancient Egyptian theology and Plato's two-world metaphysics was likely influenced by his 13 years of study in Egypt.

Related reading: Theories of Change and Constancy; Theories of Primal Substance and Cause; Theories of Time; Plato's Debt to Ancient Egypt; The Story of Ontology