  The most enigmatic aspect of consciousness is the fact that it is felt, as a
subjective sensation. The theory proposed here aims to explain this particular
aspect. The theory encompasses both the computation that is presumably involved
and the way in which that computation may be realized in the brain's
neurobiology. It is assumed that the brain makes an internal estimate of an
individual's own evolutionary fitness, which can be shown to produce a special,
distinct form of causation. Communicating components of the fitness estimate
(either for external or internal use) requires inverting them. Such inversion
can be performed by the thalamocortical feedback loop in the mammalian brain,
if that loop is operating in a switched, dual-stage mode. A first
(nonconscious) stage produces forward estimates, whereas the second (conscious)
stage inverts those estimates. It is argued that inversion produces another
special, distinct form of causation, which is spatially localized and is
plausibly sensed as the feeling of consciousness.
