  A hypothesis is presented that non-separability of degrees of freedom is the
fundamental property underlying consciousness in physical systems. The amount
of consciousness in a system is determined by the extent of non-separability
and the number of degrees of freedom involved. Non-interacting and feedforward
systems have zero consciousness, whereas most systems of interacting particles
appear to have low non-separability and consciousness. By contrast, brain
circuits exhibit high complexity and weak but tightly coordinated interactions,
which appear to support high non-separability and therefore high amount of
consciousness. The hypothesis applies to both classical and quantum cases, and
we highlight the formalism employing the Wigner function (which in the
classical limit becomes the Liouville density function) as a potentially
fruitful framework for characterizing non-separability and, thus, the amount of
consciousness in a system. The hypothesis appears to be consistent with both
the Integrated Information Theory and the Orchestrated Objective Reduction
Theory and may help reconcile the two. It offers a natural explanation for the
physical properties underlying the amount of consciousness and points to
methods of estimating the amount of non-separability as promising ways of
characterizing the amount of consciousness.
