what is the ontological commitment?
A theory of ontological commitment is a theory that tells us when we are committed to the existence of certain entities. Why do we want a theory of ontological commitment? Applying such a theory gives us a way to move from sentences commonly accepted as true to more contentious claims regarding what there is in the world. If we can show how to move from a list of true claims to a list of what exists, we can anchor our ontological claims on firm ground.
The most familiar theory of ontological commitment is that offered by Quine in his "On what there is" (1948). It may fairly be called the received view of ontological commitment. In effect, it is a combination of a criterion of ontological commitment and an account of that to which the criterion applies.
The criterion itself is quite simple. A sentence S is committed to the existence of an entity just in case either (i) there is a name for that entity in the sentence or (ii) the sentence contains, or implies, an existential generalization where that entity is needed to be the value of the bound variable. In other words, one is committed to an entity if one refers to it directly or implies that there is some individual which is that entity.
todo: Quine, W. V. O. (1948). "On What There Is." Review of Metaphysics 2: 21-38. Reprinted 1953 in his From a Logical Point of View. Cambridge, Mass.: Harvard University Press.