Contrary to received opinion among philosophers, psychologists, and neuroscientists, conscious duality as a principle of brain organization is neither incoherent nor demonstrably false. The present paper begins by reviewing the history of the theory and its anatomical basis and defending it against the claim that it rests upon an arbitrary decision as to what constitutes the biological substratum of mind or person.

It then moves on to provide a dynamic model for double consciousness in vertebrate brain organization, giving an evolutionary account that explains why, although each of the two cerebral hemispheres benefits from sensory input from the other for representation of the ipsilateral half of corporeal and extracorporeal space, it was important that conscious experience be confined to each rather than spanning the two. This interhemispheric duplication effect for sensory representation has been known for years but hitherto considered mysterious, or ignored on grounds that integration must be achieved at a higher level of processing.

The paper then attempts to resolve a puzzle about split-brain patients in testing situations, namely why it is that in spite of the speaking hemisphere's denial of any independent perception and agency in the mute hemisphere, which would explain its role in cross-cuing, the latter never seems to resent this, but instead continues to be cooperative and helpful. It is suggested that on the hypothesis of mental duality this is understandable, for the nonverbal hemisphere would have known prior to the surgery that it is not generating linguistic behavior.

Finally, the essay examines two kinds of bitemporal defects, one due to callosal and the other to chiasmal disruptions. On the present theory a bitemporal defect should be demonstrable in the former case when both eyes are open, because in the absence of a corpus callosum and other forebrain commissures the interhemispheric duplication effect is abolished; in the latter case interhemispheric duplication is preserved, and so the defect should be demonstrable only by testing each eye independently. This is indeed what the evidence indicates, so it appears that, contrary to the prevalent view, the function of the corpus callosum is not to integrate and unify conscious experience between the hemispheres but to duplicate this, in accord with the model of mental duality.