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In the introduction to this handsomely bound book, the author suggests that it is an appropriate time for an explanation of how the dynamics of a human brain can be accounted for by computer science. His title is motivated by Erwin Schrödinger's enormously influential "What is life?" which launched the field of evolutionary biology by inducing both Francis Crick and James Watson to successfully seek the molecular basis of biological evolution, but the analogy is strained for several reasons.
Schrödinger's book is less than 100 pages in a current edition, while Baum's is about five times as long. In the context of Schrödinger's lifelong interest in biological problems and based on a series of three public lectures that he presented to the Irish intelligentsia in 1943 (as one of his statutory duties as the founding director of the Dublin Institute of Advanced Studies), "What is Life?" is a classic example of his exceptional expository skill---in a second language, no less---whereas Baum's book would have profited from another round of copy-editing. But the most striking difference between these two titles lies in the cogency of their respective contents.
Although Max Delbrück and his colleagues had used measurements of mutation rates of fruit flies under X-radiation to show that their genes were necessarily of molecular dimensions in the mid-1930s, the implications of these data were unnoticed by the literate world of the mid-1940s. Thus Schrödinger's public lectures were newsworthy, being favorably noted by Time magazine in the spring of 1943, and his subsequent book---after some difficulties with an Irish publisher and the Roman Catholic Church over the religious implications of his ideas---went on to sell over 100,000 copies for Cambridge University Press, with translations into seven languages. Is there a similar communications gap in our current understanding of the nature of thought?
Noting his background in computer science, one mightclassify Eric Baum among those who believe that ``our souls are software'', but this is not quite fair. Although he states that ``the obvious inability of present-day computer science to account for [the brain's behavior] is no reason at all for doubting that they can be accounted for by computer science,'' the intellectual perspectives of "What is Thought?" are broader than this assertion seems to suggest. The book begins with several interesting chapters on the nature of computation (I particularly liked the presentation of the traveling-salesman problem), which include discussions of the importance of making decisions at the level of semantics, the Turing test, properties of neural nets, hill climbing in a fitness landscape, among several other relevant topics. These discussions lead into the author's central thesis that the mind, like all efficient computer programs, is necessarily modular. In other words, each aspect of the brain's dynamics comprises several subroutines, which presumably can be further broken down into hierarchical structures of nested activities, and he discusses several permutations of this important concept. Curiously, Baum's otherwise comprehensive list of references does not include Donald Hebb's seminal and classic work, in which the notion of ``cell assemblies'' (which are dynamically self-sufficient modules of neurons) was first suggested over a half-century ago. As a psychologist, Hebb aimed to ``bridge the long gap between the facts of psychology and those of neurology,'' and coming at about the same time as the development of the digital computer, his formulation has provided the basis for many numerical studies starting in the 1950s and continuing to the present day which are in accord with a growing body of electrophysiological data. Setting this quibble aside, Baum offers compelling psychological evidence for the modular structure of mind and provides his readers with an interesting and informative account of how the structure of our thinking may have developed over the course of biological evolution, with particular attention paid to computational constraints on the development of learning mechanisms. Importantly, his perspectives are broader than those of many of his colleagues, as he asserts that the ``whole program'' of a brain's dynamics includes the ``complex society'' in which it is embedded. Indeed, the author's evident humility in the face of awesome intricacy of mental activity is, to me, one of the more appealing aspects of "What is Thought?"
The often suggested possibilities for quantum computation are discussed in some detail, along with an analysis of the widely noted example of ``Schrödinger's cat'' which was originally proposed to emphasize the difficulties of applying ideas developed for atomic dynamics to complex macroscopic systems. Considering that a quantum computer---if it is at all possible to construct one---must be carefully isolated from structural irregularities and operated near absolute zero of temperature, Baum joins the majority of physical scientists in concluding that it is ``highly unlikely that quantum computation is relevant to the mind.''
Eric Baum has a dog, and---like most of us dog owners---he is convinced that his pet is conscious, but he goes on to assert that ``we do not need to posit new qualitative modes of thinking to explain human advance over animals. To my mind, the difference between human intelligence and animal intelligence is straightforwardly explainable by cumulative progress once there is the ability to communicate programs.'' Here, again, Baum could profit from reading Hebb's book, which contains but a single mathematical expression, namely A/S. This parameter represents the ratio of the associative area (A) of a mammalian neocortex to its sensory area (S), and it becomes greater as one progresses from rats through dogs to humans. A related physiological parameter---with profound significance for the ease and rate at which modules (or cell assemblies) can switch on and off---is the percentage of inhibitory intercortical neurons, varying as follows: rabbit (31%), cat (35%), monkey (45%), human (75%) . Of course, these relative differences may be examples of the ``cumulative progress'' to which Baum refers.
In a penultimate section, Baum discusses the question of free will, noting that ``our decisions look, from any reasonable perspective short of knowing the exact state of our brains and simulating them in detail, like they are introducing genuinely new information.'' In reaching this conclusion, he may be confused by the continuing tendency of many scientists to overlook a phenomenon called ``sensitive dependence on initial conditions'' first studied by the eminent French mathematician Henri Poincaré and widely observed nowadays by those who study nonlinear dynamic phenomena (chaos theory). As Poincare` famously put it over a century ago:
"If we knew exactly the laws of nature and the situation of the universe at the initial moment, we could predict exactly the situation of that same universe at a succeeding moment, but even if it were the case that the natural laws had no longer any secret for us, we could still only know the initial situation approximately. If that enabled us to predict the succeeding situation with the same approximation, that is all we require, and we should say that the phenomenon had been predicted, that it is governed by laws. But it is not always so; it may happen that small differences in the initial conditions produce very great ones in the final phenomena. A small error in the former will produce an enormous error in the latter. Prediction becomes impossible, and we have the fortuitous phenomenon."
For an author who bases many of his conclusions on close mathematical reasoning and offers a theory that purports to be ``capable of explaining everything,'' the implications of these ``fortuitious phenomena'' should be carefully digested.