American Scientist

A Mind So Rare: The Evolution of Human Consciousness. Merlin Donald. xvi + 371 pp. W. W. Norton, 2001. $28.95.

Philosophers have recently defended various skeptical views about the nature of consciousness. They argue that consciousness cannot be understood or that even if the neurophysiology of conscious experience is understood, we will not have come close to understanding consciousness itself. One prominent contemporary philosopher argues that evolutionary biology is irrelevant to understanding consciousness and that the problems about consciousness that neuroscientists consider tractable are "easy" to solve. Neuropsychologist Merlin Donald is exasperated with all this philosophical naysaying. He puts a robust notion of consciousness at center stage in human mental life and goes on to defend a theory of how our form of consciousness evolved to help make us unique.

The current literature on consciousness contains two different skeptical views: There are philosophers, such as those mentioned above, who believe that consciousness cannot be understood (or believe subtly different versions of this view), and there are philosophers, psychologists and neuroscientists who believe that experimental work has shown that consciousness is fleeting and ephemeral at best. This latter group, whom Donald calls Hardliners, "don't deny consciousness, but they trivialize it." They take the experimental work on consciousness to have proved it to play a limited role in mental governance. Daniel Dennett's idea that our perception that we are consciously in charge of our decisions and behavior is simply an illusion makes him an archetypal Hardliner. Donald wants to pursue a third option: the view that consciousness is a robust and interesting component of human mental life. He emphasizes the immense gap between himself and the Hardliners: "In my world," he tells us, "consciousness is king." He argues that an account of consciousness is the key to understanding much human behavior and that the peculiar human form of consciousness was a prerequisite for the evolution of our linguistic abilities, which set us apart from other animals.

Donald argues that the term "consciousness" should be used to designate three different "levels of awareness": selective binding, short-term control, and intermediate- and long-term governance. He defines binding as "the theoretical basis of object perception or, more accurately, the neural means of obtaining perceptual unity." The fact that humans can perceive objects distinguishes us from many other creatures. Short-term control is based in short-term memory and allows for the learning of complex procedures. Intermediate- and long-term governance involve both the long-term regulation of thought and behavior and the evaluative capacity of checking on or scrutinizing the results of short-term control. He hypothesizes that all three levels of awareness evolved together, possibly as a complex adaptation. But even this complex cluster of awareness levels is not all there is to human consciousness, according to Donald. He adds that humans are the only organisms that exist within a "web of meaning" and that this web, also referred to as culture, is what complements and completes the internal attributes of consciousness.

This complex position is defended in various ways throughout the book. Donald generates some credibility for his view by criticizing members of the camps mentioned above as well as presenting various sources of evidence for the particulars of his position. He tackles the Hardliners by taking a look at experimental work in both psychology and neuroscience. For example, he discusses the case described by Alexander Luria of a gunshot victim given the pseudonym Zasetsky, who lost much of his occipital and parietal cortex on the left side. As a result, his experience was fragmented and he lacked most of his capacity for short-term memory. Donald argues that Zasetsky has lost most of what Hardliners take consciousness to be—the passive monitoring of immediate experiences. The problem is that Zasetsky managed to write a detailed diary about his experiences and to control various aspects of his fragmented existence. Donald argues that this case provides evidence for a kind of consciousness that is "a strategic device without which online, real-time decisions could not be made, reconstructions of ideas and memories could not take place, and new plans could not be assembled."

Readers from the Hardliner camp will likely not be easily swayed by Donald's arguments. He tends to assume that everyone concerned can agree to his changing of the terms of the debate and hence that the term "consciousness" must refer to all the mental processes he ascribes to it. For some, accepting this would be giving up too much. For example, Donald recommends going back to a homuncular notion of the controlling center of consciousness, which is a position that Daniel Dennett and others have argued strenuously against and are hardly likely to take as a return to uncontested common ground.

Donald is more persuasive when he canvasses evidence from comparative neuroanatomy for his idea that human consciousness differs from that of apes and other mammals along numerous dimensions. For example, he points out that human brains are larger than ape brains as a result of increases in size in the tertiary regions of the cortex (and of the hypothalamus), rather than as a result of a uniform increase in the size of all brain regions. So although apes may share some of the three levels of consciousness presented above, they have hardly any of the functions associated with the executive systems of planning, self-representation and the long-term supervision of action whose neural basis is in the tertiary cortex. Although Donald is more persuasive in these parts of the book, it must be noted that many of his observations here are not original.

Unfortunately, although Donald's book is filled with interesting examples and imaginative theorizing, his overall position is not characterized sharply enough to present the reader with a clear choice between him and his Hardliner opponents. Still, he has succeeded in making work from the clinical and experimental neurosciences accessible to a wide range of readers and points to some fruitful future areas of exchange in the interdisciplinary study of consciousness.—Stephen M. Downes, Philosophy, University of Utah