We feel therefore we are. Conscious sensations ground our sense of self. They are essential to our idea of ourselves as psychic beings: present, existent, and mattering. But is it only humans who feel this way? Do other animals? Will future machines? To answer these questions we need a scientific understanding of consciousness: what it is and why it has evolved. Nicholas Humphrey has been researching these issues for fifty years. In this extraordinary book, weaving together intellectual adventure, cutting-edge science, and his own breakthrough experiences, he tells the story of his quest to uncover the evolutionary history of consciousness: from his discovery of blindsight after brain damage in monkeys, to hanging out with mountain gorillas in Rwanda, to becoming a leading philosopher of mind. Out of this, he has come up with an explanation of conscious feeling - ''phenomenal consciousness'' - that he presents here in full for the first time. Building on this theory of how phenomenal consciousness is generated in the human brain, he turns to the morally crucial question of whether it exists in non-human creatures. His conclusions, on the evidence as it stands, are radical. Contrary to both popular and much scientific opinion, he argues that phenomenal consciousness is a relatively recent evolutionary innovation, present only in warm-blooded creatures, mammals and birds. Invertebrates, such as octopuses and bees, for all their intelligence, are in this respect unfeeling zombies. And for now, but not necessarily for ever, so are man-made machines.