Full Idea
Three degrees necessity in logic or semantics: first and least is attaching a semantical predicate to the names of statements (as Nec '9>5'); second and more drastic attaches to statements themselves; third and gravest attaches to open sentences.
Gist of Idea
Necessity can attach to statement-names, to statements, and to open sentences
Source
Willard Quine (Three Grades of Modal Involvement [1953], p.158)
Book Reference
Quine,Willard: 'Ways of Paradox and other essays' [Harvard 1976], p.158