Each symbol a is associated with its supplement a which satisfies the **equivalences** a+a = i, aa = o, the latter of which means that a and a have no region in common.

If certain **equivalences** between volumes in different countries are stated here, it`must be plainly understood that they are only known to be approximate results, and not to give a certain basis for any theories of derivation.

If we were to say that on his view the essential step must be the establishment of identities or **equivalences**, we should probably be doing justice to his doctrine of numerical reasoning, but should have some difficulty in showing the application of the method to geometrical reasoning.