For the question of why there is something and not nothing, see being. The modern treatment of existence in the theory of quantification is sometimes put by saying that existence is not a predicate. The idea is that the existential quantifier is itself an operator on a predicate, indicating that the property it expresses has instances. Existence is therefore treated as a second-order property, or property of properties. In this it is like number, for when we say that there are three things of a kind, we do not describe the things (as we would if we said there are red things of the kind), but instead attribute a property to the kind itself. The parallel with numbers is exploited by Frege in the dictum that affirmation of existence is merely denial of the number nought. A problem for the account is created by sentences like ‘This exists’, where some particular thing is indicated. Such a sentence seems to express a contingent truth (for this might not have existed), yet no other predicate is involved. ‘This exists’ is therefore unlike ‘Tame tigers exist’, where a property is said to have an instance, for the word ‘this’ does not locate a property, but only an individual. Possible worlds seem able to differ from each other purely in the presence or absence of individuals, and not merely in the distribution of exemplifications of properties.