Every S is P ;
Every not-P is not-S ;
Nothing is both S and not-P ;
Everything is either P or not-S ;
and these four propositions have been shewn to be equivalent to one another.
[155] Compare Mrs Ladd Franklin in Mind, January, 1890, p. 87.
If in the above propositions we now write S for P, we have the following:
Every S is S ;
Every not-S is not-S ;
Nothing is both S and not-S ;
Everything is either S or not-S.