---
tag: 9Q0KH6C5X3
kind: concept
name: No-classes theory
sources:
  - wikipedia: en:43580500@r1360947732
    locator: Glossary of set theory, section N
    note: adopted
    mention: wikipedia://en/43580500#S8BK
    sections: [Summary]
inventory: [HD337TFFN7]
---

## Summary

A theory due to Bertrand Russell, and used in his *Principia Mathematica*, according to which [[sets|0BSJ7RQ9BW]] 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.)