Russell doesn't
really wish to say that two classes are similar if they are
correlated.
He needs a correlation which isn't a grossly material one, which
in fact is the possibility of a material correlation.
And that means that when he says A & B are
1-1
correlated it really means they can be, or it
makes sense to say that they are.
So we are driven to consider cases in which it makes sense & such
in which it makes no sense to say that
91
two classes are
1-1
correlated.
What is the condition of their being 1-1 correlatable.