What is existence? A short tour of ontology

Open a binary in a disassembler and the oldest question in philosophy shows up in miniature. There is a function at 0x401136. Does it exist? Obviously: there are bytes at that address. But is "the function" in the file, or only in our reading of it? And what about the name the programmer gave it, which the compiler stripped long ago?

Ontology is the part of philosophy that asks what there is, and what it is to be. The word joins Greek on (that which is) and logos (account). It became a technical term in the 17th century, but the questions are much older. This post is a fast tour with a timeline, a map and a diagram. Later posts slow down at one stop: see Spinoza: God, or Nature.

Parmenides of Elea (early 5th century BCE) wrote a poem in which a goddess shows the narrator two paths: what is and what is not. The second cannot be thought or spoken, so only the first remains. From this he drew a startling chain: what is cannot come from nothing, cannot perish, cannot change, and is all of a piece. The senses show a world of change, so the senses must deceive.

Heraclitus of Ephesus, around the same time, took the opposite side. For him the world is a fire-like process held together by a hidden order, the logos. His best-known image is the river: different waters flow on those who step into the same river. Later tradition compressed this into "everything flows". Scholars still argue about what exactly Parmenides meant (strict monism, or a claim about what can be thought and said), so take the textbook version as a first sketch.

Both camps are alive today. Physics loves things that stay put (conservation laws) and also processes that never stop (heat flowing, fields oscillating).

Plato tried to keep both. Particular beautiful things come and go, but Beauty itself does not, and it is what makes them beautiful. These unchanging Forms are what is fully real; the visible world is a changing, derivative copy. Plato himself pointed out problems with the picture in his dialogue Parmenides. Many mathematicians still talk like Platonists when they say a theorem about primes is discovered, not invented.

Aristotle brought the real thing back down to earth. There are no separate Forms. What is primarily real is the individual substance: this horse, this person, a compound of matter and form. And "being" is said in many ways. Besides substance, we say a thing has a size, a quality, a place, and so on. His Categories list ten such ways.

The struct is only an analogy (for Aristotle the substance is not a bag of fields), but it shows the idea: one thing, many ways of being said of it.

Ibn Sina, known in Latin as Avicenna (c. 980-1037), asked a sharp question. You can fully understand what a triangle is, or a phoenix, without knowing whether any exists. So existence is not part of the definition. If a thing's essence does not include its existence, then its existence must come from outside it. Follow the chain of causes upward and it must end in a being whose essence is to exist: the Necessary Existent. Thomas Aquinas took over the distinction in On Being and Essence. Scholars still debate how Avicenna understood it, whether as a real addition to a thing or as a distinction made by the mind, and Averroes criticised it.

Reverse engineering is Avicenna's problem run backwards. The thing certainly exists, you have it in hand, and your job is to recover what it is.

In the Meditations (1641) René Descartes doubted everything doubtable and found one certainty: whenever he thinks "I am, I exist", it is true. Existence is first known from the inside. He then split reality into thinking substance and extended substance, a split that Spinoza would later try to repair. Descartes also revived an old argument, going back to Anselm (1077-78): God is a perfect being, existence is a perfection, so God exists.

Immanuel Kant replied in the Critique of Pure Reason (1781) that "being is evidently not a real predicate". A hundred real thalers contain no more than a hundred possible ones; if they did, the concept would not describe the real coins. The difference shows up in my purse, not in the concept. Saying a thing exists does not add a property. It says the concept has an instance. David Hume had hinted at this in 1739, and the logicians Gottlob Frege (1884) and Bertrand Russell (1905) turned it into formal logic, where "unicorns do not exist" means: nothing satisfies the description "unicorn".

Martin Heidegger opened Being and Time (1927) by claiming philosophy had forgotten the question. We treat "being" as obvious and study beings (things) instead. He asked about Being itself, and started from the one entity for whom its own being is an issue: us, which he called Dasein. We are not detached observers first; we are already in a world of tools and tasks. A hammer shows up as something to hammer with long before it shows up as a lump of matter. Anyone who has understood a function by what it does in the system, not by staring at its bytes, has made Heidegger's move. A note of honesty: he joined the Nazi party in 1933, and scholars disagree about how far that is connected to his philosophy.

Jean-Paul Sartre turned the essence/existence pair into a slogan. In a 1945 lecture he said that a paper-knife has an essence before it exists, because the craftsman has the concept first. If humans were made by a designer, the same would hold for us. Sartre, an atheist, says there is no designer, so a person first exists, then defines themselves by what they choose. Critics reply that biology and society give us far more given structure than he allowed (see Evolution, illustrated).

W. V. O. Quine brought the question back down. In "On What There Is" (1948) he noted that "what is there?" can be answered in one word, everything, and the real argument is over what falls under it. His rule: to be is to be the value of a variable. Your theory commits you to whatever it must quantify over to come out true. If your best science needs electrons and numbers, you owe those a place in your ontology. Rivals disagree: Carnap said such questions are choices of language, and David Lewis argued that other possible worlds are real.

A binary has its own ontological commitment: the import table, the list of things that must exist out there for it to run.

Three habits survive the tour. Separate what a thing is from whether there is one. Ask what a claim commits you to. And notice where you are standing when you ask: the thinker, the tool user, the chooser.

One name did not fit the map: Spinoza made the strangest move of all. For him there is exactly one thing whose essence involves its existence, and it is everything. That is the subject of Spinoza: God, or Nature.