Full Idea
Russell held that classes can be reduced to propositional functions.
Gist of Idea
Classes can be reduced to propositional functions
Source
report of Bertrand Russell (Mathematical logic and theory of types [1908]) by Robert Hanna - Rationality and Logic 2.4
Book Reference
Hanna,Robert: 'Rationality and Logic' [MIT 2006], p.38
A Reaction
The exact nature of a propositional function is disputed amongst Russell scholars (though it is roughly an open sentence of the form 'x is red').