The existence of an internal relation between possible states of affairs expresses itself in language by an internal relation between the propositions presenting them.
4.1251 Now this settles the disputed question "whether all relations are internal or external".
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
"(x) : aRx . xRb",
"(x, y) : aRx . xRy . yRb",
(If b stands in one of these relations to a, I call b a successor of a.)