ABSTRACT
The equality of two classes A and B will be denoted by the symbol = A B. The sign = denotes class equality and is to be
Ci Cl distinguished from semantical identity. Classes are equal if they have the same elements. However, this does not imply that the classes are identical. For example, when one has = (man) (featherless biped), the concepts man and featherless Cl biped are not identical. The notion of the identity of two equal classes has raised problems which have long troubled logicians. Recently the so-called axiom of extensionality which assures the identity of two equal classes 3 has been generally accepted. This axiom has proved to be superfluous because one can confine oneself to extensional classes.4
