Capek, Milic.  “Discussion: Note about Whitehead's Definitions of Co-Presence.”  Philosohpy of Science 24 (Jan. 1957): 79-86.

Abstract

Whitehead's two definitions of co-presence are analyzed.  In The Concept of Nature, co-presence is defined as a dyadic relation holding between two events in one instantaneous space and is illustrated by one misleading example.  Two years later, in The Principle of Relativity, it is defined as a relation between the event a and the whole four-dimensional region separating the causal past from the causal future of the same event.  This is the same what Eddington called "elsewhere."  Only in his later writings Whitehead substituted a less misleading term "contemporary" to that of "co-present" which is easily confused with "co-instantaneous"; he also then correctly stressed the causal unrelatedness of the corresponding events.  [Abstract from The Philosopher’s Index]