On What There Is

W.V.O. Quine's 1948 essay is one of the most influential texts in 20th-century metaphysics and logic. Quine shows how claims about what "exists" can be analyzed by examining which entities a language's logical structure actually commits us to — summarized in his famous thesis that "to be is to be the value of a bound variable." The essay was originally published in The Review of Metaphysics (1948) and later reprinted in the collection "From a Logical Point of View: 9 Logico-Philosophical Essays".

Although the essay is nearly eighty years old, its argument is still the starting point for how ontology is discussed in both philosophy and AI — several later texts, including Gruber and Smith on this list, build directly on Quine's foundational question. The essay is freely available to read online, so you don't need to buy the whole collection to read it.