Niels Bohr proposed that one fundamental insight of quantum physics was that some phenomena or systems could be described in two or more mutually exclusive ways and that it would be a mistake to pick one description as the “right one” – both could be accurate.
This violates the logical dictum of the excluded middle, in a sense, since it suggests that when we ask if a system is A or B, the answer is “both”. It is a radical view, and just how radical is illustrated by physicist Frank Wilczek’s use of another example: legal liability.
Wilczek suggests that humans can be described as physical systems wholly determined by physical laws, or acting intentional agents with motives and responsibility – and that both descriptions are accurate representations. It would be a mistake, then, to argue that we have no legal responsibility because we can be described as physical systems, because we can equally well be described as acting, intentional agents.
This feels like cheating – and at least my inner high-school philosopher wants to know “but which is it?” — the challenge here is if we accept the lack of a singular ontological ground truth we suddenly seem to be on a slipper slope to Feyerabend-land, where everything is permissable.
This is an interesting problem – it echoes of Dostoyevsky’s observation that if God is dead, then all is permitted — the lack of a single foundational layer of reality, the lack of a hierarchy of reality, seems to unmoor us from truth. But maybe the answer to that is in the “mutually exclusive”? If two descriptions overlap in any way, then they do not count — what we need to have developed a complementarity is to have a description that is excludes the others.
We then end up with a new meta-layer of ontology: all the mutually exclusive descriptions of a phenomenon that we can devise. This is intriguing, because it suggests that the idea of the model has a much more foundational role in describing reality than we may have guessed; rather than just abstract things away, complementarity suggests that the world is made of mutually exclusive models.
One could also imagine a pragmatic version of a complementarity metaphysics: the time spent reducing sufficiently mutually exclusive descriptions to a single one is not well-invested; it is much better to seek ways of using these mutually exclusive descriptions and judge them on their usefulness.