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:
“(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.”