The indefinables of logic must be independent of each other. If an indefinable
is introduced, it must be introduced in all combinations in which it can occur.
We cannot therefore introduce it first for one combination, then for another;
e.g., if the form xRy has been introduced, it must henceforth be
understood in propositions of the form aRb just in the same way as in
propositions such as (∃x,y). xRy & others. We must not introduce it
first for one class of cases, then for the other; for it would remain doubtful if
its meaning was the same in both cases, & there would be no ground for
using the same manner of combining symbols in both cases. In short, for
the introduction of indefinable symbols & classes combinations of symbols the same holds,
mutatis mutandis, that Frege has said for the introduction of symbols
by definitions.