These two perspectives are mutually exclusive, since the identity principle, by which one is defined as constant, is precisely what has to be treated as varying in defining the other. They cannot therefore be employed simultaneously. But precisely because of this mutual exclusiveness, both forms of identification reinforce each other as complementary negatives. In other words, variation, as such, is inconceivable, if one cannot assume identities with respect to which something is changing.

Niklas Luhmann, Trust and Power, loc. 117