4.1252
Series which are ordered by internal relations I call formal series. The series of numbers is ordered not by an external, but by an internal relation. Similarly the series of propositions “aRb”, “(∃x):aRx.xRb”, “(∃x,y):aRx.xRy.yRb”, etc. (If b stands in one of these relations to a, I call b a successor of a.)