What about the second relatum? What is it that theories are committed to? Sometimes we say that a theory is committed to a particular entity. For example, we say that the sentence ‘Socrates exists’ (or, in our austere language, ‘∃x Socratizes(x)’) is committed to Socrates. But more often we speak of ontological commitment to some entities in the plural, or to some kind of entity.  For example, we speak of theories being committed to universals, or numbers, or electrons. But ontological commitment to some entities plurally, or some kind of entity, is clearly not the same as ontological commitment to each of the entities singly. Consider a theory that contains ‘∃x Elephant(x)’. It is ontologically committed to elephants, or to the kind elephant, but not to any particular elephant: no particular elephant need be the value of the variable ‘x’ in order for the sentence to be true. A theory is committed to Ks, or a kind K, just in case the values of the variables must include at least one K in order for the theory to be true, in which case the existence of Ks is in some sense entailed by the theory. (More on different relevant senses of entailment in §1.5 and §2 below.) 