In all traditional philosophy there comes a heading of 'modality', which discusses necessary, possible, and impossible as properties of propositions, whereas in fact they are properties of propositional functions. Propositions are only true or false.
If you take 'x is x', that is a propositional function which is true whatever 'x' may be, i.e. a necessary propositional function. If you take 'x is a man', that is a possible one. If you take 'x is a unicorn', that is an impossible one.
Propositions can only be true or false, but propositional functions have these three possibilities..
Source:Bertrand Russell: The Philosophy of Logical Atomism, 1918.
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