Tractatus Logico-Philosophicus by Ludwig Wittgenstein + comments  by Maarten Maartensz   

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.