mathematics requires.
so far as I know.
only two other primitive propositions.
the one that material implication is a relation.
the orther that e ( the relation of a term to a class to which it belongs ) is a relation.
We can now develop the whole of mathematics withont further assumptions or indefinables.
Certain propositions in the logic of relations deserve to be mentioned,
Since they are important and it might be doubted whether they were capable of formal proof.
If be any two classes ,
there is a relation R the assertion of which between any two terms x and y is equivalent
to the assertion that x belongs to u and y to v .
if u be any class which is not null,
there is a relation which all its terms have to it ,
and which holds for no other pairs of terms.
If R be any relation, and u any class contained in the class of referents with respect to R,