A good deal has already been written on the possible relevance of Niels Bohr's principle of ‘complementarity’ to various theological issues. Bohr, himself, suggested that the concept might be useful in discussions concerning the relation of intra-mundane causality and divine providence, or that of human freedom and divine sovereignty. These suggestions have been taken up and developed by C. A. Coulson and D. M. Mackay, but they have also seriously been criticized, notably by I. G. Barbour. The principal difficulty encountered in regarding God and the world as ‘complementary’, in Bohr's sense of the term, is that Creator and creature are generally thought to be two distinct ‘entities’, in Christian ‘theism’, rather than two ‘modes’ of a single entity as ‘wave’ and ‘particle’ are two ‘complementary’ modes of an atomic object in physics.