Ford, Lewis S.  "Efficient Causation Within Concrescence."  Process Studies 19, no. 3 (Fall 1990): 167-80.

Abstract

Whitehead's early decision to treat the 'transition' between occasions in terms of efficient causation, and the 'conscrescence' within occasions in terms of final causation led to a very thin theory of efficient causation as 'concrescence' incorporated more and more of the functions of 'transition.'  I propose we understand efficient causation in terms of the activity of physical prehension both between and within occasions.  Robust interaction among physical prehensions within occasions is not really possible without concrescent elimination, but this becomes possible with the introduction of subjective aim.  If so, Whitehead could revive the old term 'efficacity' to mean an active efficient causation within concrescence.  [Abstract from The Philosopher's Index]