Proper Classes
Kees Doets
Abstract:
A formula $\Phi(x)$ in the one nonĀlogical 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 nearĀRussell.