MacLachlan, D. L. C.  "Whitehead's Theory of Perception."  Process Studies 21, no. 4 (Winter 1994): 227-30.

Abstract

Whitehead supplements the awareness of sense-data, which dominated the discussion of perception in his day, and which he calls "experience in the mode of presentational immediacy", with a second type, "experience in the mode of causal efficacy".  This makes possible symbolic reference beyond sense-data, especially if, as I argue, presentational immediacy develops in the subject in order to map clearly the physical reality dimly discerned through the other mode.  [Abstract from The Philosopher's Index]