5.233
Operations cannot make their appearance
before the point at which
one proposition is generated out of another
in a logically meaningful
way; i.e. the point at which the logical
construction of propositions
begins.

This seems to be so
in the Principia Mathematica and there makes
sense. Then again, one may start from the notion
of a function rather than of a proposition e.g.
as in combinatory logic.