Buchler, Justus.  “On a Strain of Arbitrariness in Whitehead's System.”  The Journal of Philosophy 66 (Oct. 1969): 589-601.

Abstract

There are two incompatible trends in Whitehead's system.  Trend I enunciates categories or "types of existence" which differ only in scope or comprehensiveness.  Trend II assigns to these types grades of reality, with actual occasions singled out as the "really real" and "most concrete" beings.  It is shown that trend II introduces a serious dimension of arbitrariness into a system that prides itself on "reasons" and on explanatory adequacy in philosophy.  Stress is laid on Whitehead's notions of concreteness and degree of concreteness, which are not clarified by the notion of concrescence, and which prove to be surprisingly opaque.  [Abstract from The Philosopher’s Index]