No-classes theory

Summary

A theory due to Bertrand Russell, and used in his Principia Mathematica, according to which sets can be reduced to certain kinds of propositional function formulae. (In Russell's time, the distinction between "class" and "set" had not been developed yet, and Russell used the word "class" in his writings, hence the name "no-class" or "no-classes" theory is retained for this historical reason, although the theory refers to what are now called sets.)