Ontological commitment

In the philosophy of language and metaphysics, an ontological commitment is said to be necessary in order to make a proposition in which the existence of one thing is presupposed or implied by asserting the existence of another. We are “committed” to the existence of the second thing, even though we may not have expected it, and may have intended to assert only the existence of the first. The kind of secondary entities in question are typically abstract objects such as universals, sets, classes, or fictional objects.

Background
The sentence “Napoleon is one of my ancestors” asserts only the existence of two individuals (i.e., Napoleon and the speaker) and a line of ancestry between them. The fact that no other people or objects are mentioned seems to limit the “commitment” of the sentence. However, it is well-known that sentences of this kind cannot be interpreted in first-order logic, where individual variables stand for individual things. Instead, they must be represented in some second-order form. In ordinary language, such second-order forms use either grammatical plurals or terms such as “set of” or “group of”.

For example, the sentence involving Napoleon can be rewritten as “any group of people that includes me and the parents of each person in the group must also include Napoleon,” which is easily interpreted as a statement in second-order logic (one would naturally start by assigning a name, such as G, to the group of people under consideration). Formally, collective noun forms such as “a group of people” are represented by second-order variables, or by first-order variables standing for sets (which are well-defined objects in mathematics and logic). Since these variables do not stand for individual objects, it seems we are “ontologically committed” to entities other than individuals — sets, classes, and so on. As Quine puts it,

"the general adoption of class variables of quantification ushers in a theory whose laws were not in general expressible in the antecedent levels of logic. The price paid for this increased power is ontological: objects of a special and abstract kind, viz. classes, are now presupposed. Formally it is precisely in allowing quantification over class variables α, β, etc., that we assume a range of values for these variables to refer to. To be assumed as an entity is to be assumed as a value of a variable. (Methods of Logic p. 228)"

Another statement about individuals that appears “ontologically innocent” is the well-known Geach–Kaplan sentence: Some people admire only each other.

Ontological innocence
Many philosophers dispute whether we are committed to such entities at all. They argue that all assertions are “ontologically innocent” &mdash; that they are committed only to the existence of the entities which they actually assert.

There is a considerable and growing body of literature on so-called plural reference and plural quantification. It seems counter-intuitive that a sentence such as “some people admire only each other” commits us to the existence of anything but people. Advocates of ontological innocence see in the grammatical plural simply another way to refer to exactly the same things that the singular form commits us to.

Interpretations of negative existential statements have proved more difficult. See empty names for an overview of the problem.