Reality may be described in terms of external relations (entities have pre-existing properties) and in terms of internal relations (the properties are a consequence of the relation). These two ways of description are complementary. They are found in the work of different authors (Bohr, Whitehead, systems theory, Bohm, Löfgren, Prigogine). Bohr explicitly formulates a theory of complementarity, combining the two ways of description. How do other authors combine their two views of reality, is the concept of complementarity here also suitable as unifying concept? What are the implications of this complementary view on reality for the object-subject relation in epistemology? In contrast to the traditional view, where object and subject are postulated in terms of an external relation, I propose a 'complementary object-subject-relation'.