Full Idea
Traditional philosophy discusses 'necessary', 'possible' and 'impossible' as properties of propositions, whereas in fact they are properties of propositional functions; propositions are only true or false.
Clarification
A 'propositional function' is (roughly) a property or relation available for attributing to things
Gist of Idea
Modal terms are properties of propositional functions, not of propositions
Source
Bertrand Russell (The Philosophy of Logical Atomism [1918], §V)
Book Reference
Russell,Bertrand: 'Russell's Logical Atomism', ed/tr. Pears,David [Fontana 1972], p.88
A Reaction
I am unclear how a truth could be known to be necessary if it is full of variables. 'x is human' seems to have no modality, but 'Socrates is human' could well be necessary. I like McGinn's rather adverbial account of modality.