4.1252 Series which are ordered according to internal relations I call formal series.

The series of numbers is ordered not by an external but rather by an internal relation.

Equally the series of propositions:

“aRb,”

“(Ex): aRx. xRb,”

“(Ey): aRx. xRy. yRb,” etc.

(If b stands in one of these relations to a then I call b a successor of a.)

The first part confirms my comment on 4.123. The second introduces a definition of “successor.”