Niels Bohr's maxim contraria sunt complementa indicated his strong suspicion that the complementarity interpretation of quantum mechanics might someday be expanded into a generalized principle. It now appears that such a principle has been found in metastability which appears at the scale of living things. Metastability has been proposed as a principle of brain~behavior, and is captured in the extended or 'broken-symmetry' version of the HKB model of coordination dynamics. The metastable regime of coordination dynamics reconciles the tendency of specialized brain regions to express autonomy (segregation) and their simultaneous tendency to work together as a synergetic whole (integration). There is growing evidence from recent studies in the brain and behavioral sciences that the complementary nature of integrating and segregating tendencies is essential to the way human brain~minds work.