Could the human species be a gigantic evolutionary mistake? To answer that startling question Koestler examines how experts on evolution and psychology all too often write about people with an "antiquated slot-machine model based on the naively mechanistic world-view of the nineteenth century". His brillinat polemic helped to instigate a major revolution in the life sciences, yet its "glimpses of an alternate world-view" form only the background to an even more challenging analysis of the human predicament. Perhaps, he suggests, we are a species in which ancient and recent brain structures -- or reason and emotion -- are not fully coordinated. Such inbuilt deficiencies may explain the paranoia, violence and insanity that are central strands of human history. And however disturbing we find such issues, Koestler contends, it is only when we face our limitations head-on that we can hope to find a remedy.

Koestler examines the notion that the parts of the human brain-structure which account for reason and emotion are not fully coordinated. This kind of deficiency may explain the paranoia, violence, and insanity that are central parts of human history, according to Koestler's analysis of the human predicament.