"René Descartes developed the idea that human beings have a dual nature: they have a body... of material substance, and a mind, which derives from the spiritual nature of the soul. ... It is remarkable to reflect that these seventeenth century ideas were still current in the 1980s. Karl Popper... and John Eccles... espoused dualism all their lives. They agreed with Aquinas that the soul is immortal and independent of the brain. Gilbert Ryle... referred to the notion of the soul as "the ghost in the machine." Today, most philosophers of mind agree that what we call consciousness derives from the physical brain, but some disagree with Crick as to whether it can ever be approached scientifically. A few, such as Colin McGinn, believe that consciousness cannot be studied... At the other extreme, philosophers such as Daniel Dennett deny that there is any problem at all. Dennett argues much as... John Hughlings Jackson did... that consciousness is not a distinct operation of the brain; rather it is a combined result of computational workings of higher-order areas of the brain... Philosophers such as John Searle and Thomas Nagel take a middle position, holding that consciousness is a discrete set of biological processes... very complex and... more than the sum of their parts."