In spite of a resurgence in interest regarding the nature of consciousness,
some puzzling aspects of the relationship between the mind and the brain r
emain unexplained, One particularly mysterious feature of consciousness is
known in philosophy as the 'grain problem': how does the divisible and hete
rogeneous brain produce the subjectively unified and seamless mind? Some mo
dels of the brain-mind relationship, such as the one offered by Sperry, ass
ume that the brain functions hierarchically like a pyramid, with consciousn
ess mysteriously 'emerging' unified at the summit of the hierarchy. This mo
del fails to account for the manner in which both lower and higher hierarch
ical levels of the brain contribute to consciousness. In this paper, I prop
ose an alternative model of the brain-mind relationship in which the brain
functions, like all living things, as a nested hierarchy. While the specifi
c neurophysiological mechanisms behind cerebral integration require further
elucidation, the difficult philosophical problem of the difference between
the grain of the brain and the mind has a neurobiological explanation.