Our initial sensory data are always "first derivatives," statements about differences which exist among external objects or statements about changes which occur either in them or in our relationship to them. Objects and circumstances which remain absolutely constant relative to the observer, unchanged either by his own movement or by external events, are in general difficult and perhaps always impossible to perceive. What we perceive easily is difference and change — and difference is a relationship.

p. 173 - Communication: The Social Matrix of Psychiatry, 1951