Among all the major scientists of the twentieth century, Niels Bohr may have most wanted to be considered a philosopher. Bohr thought that his concept of complementarity, developed in the same weeks as Werner Heisenberg was formulating his uncertainty principle, could explain many great philosophical issues. Complementarity in the form of wave-particle duality lies at the core of the Copenhagen interpretation of quantum mechanics. Over the years, Bohr suggested complementarity could illuminate the mind/body problem, it might provide for the difference between organic and inorganic matter, and it could underlie other classic dualisms like subject/object, reason versus passion, and even free volition versus causality.

Like any educated person of his time, Bohr knew of Kant's phenomemal/noumenal dualism. He often spoke as if the goal of complementarity was to reconcile opposites. He likened it to the eastern yin and yang, and his grave is marked with the yin/yang symbol.

Bohr was often criticized for suggesting that both A and Not-A could be the case. This was the characteristic sign of Hegel's dialectical materialism. Had Bohr absorbed some Hegelian thinking? Another Hegelian trait was to speak indirectly and obscurely of the most important matters, and this was Bohr's way, to the chagrin of many of his disciples. They hoped for clarity and but got mostly fuzzy thinking when Bohr stepped outside of quantum mechanics.

Bohr would very much have liked the current two-stage model for free will incorporating both randomness and adequate determinism. He would have seen it as a shining example of his complementarity.