There is but one “I” in each us. But to say that there is one personhood per human being begs the question. That is, why are there not multiple personhoods per mind-body ensemble?

Studies of the brain performed with such techniques as magnetic-resonance imaging (MRI)^WP and positron-emission tomography (PET)^WP are helping researchers to elucidate the physical correlates of various mental activities. These technologies are mentioned because they may, at a future time, be directly helpful in the design—and, especially, in the testing—of interfaces. For example, there is an inverse correlation between a person’s localized glucose uptake—an indicator of how much energy the brain is using in a particular physical structure—and the ease with which that person uses a tested interface feature. Interface testing in the future may well make increasing use of direct measures of brain activity, but a further exploration of these methods lies outside the scope of this book.