Proper Classes
Kees Doets

Abstract:
A formula $\Phi(x)$ in the one non­logical symbol $E$ with one free variable 
$x$ is Russell if the sentence $\not \exists r \forall x (x E r \iff \Phi(x))$ 
is logically valid. This note describes a pattern common to the classical 
examples of Russell formulas, adds a couple of new ones, and constructs many 
formulas that are near­Russell.