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Darwinian thinking emphasizes selection over instruction. Some variations are "selected" by the environment over others. Adaptation to the environment is a process of selection.
Gazzaniga applies this way of thinking to the mind.
The mind is shaped by the environment, but the environment can only shape it as far as genetically-fixed parameters allow. It is more appropriate to say that the environment "selects" from the possible outcomes.
During growth, selection processes determine how a brain is wired for adult functioning.
Gazzaniga gives credit to the Danish immunologist Niels Jerne who, in 1968, was the first to argue that if a selection process also accounts for the mind: do we learn new concepts or is a concept chosen by the environment among a pre-existing array of concepts, do we create a plan of action or is an action selected by the environment from a pre-existing set of actions, do we think or is a thought selected from a vast pool of possible thoughts? Do we design our mental life, or is our mental life a continuous process of environment selection of events in our brain? Does our mind manufacture ordered thought or does it manufacture chaotic mental events that the environment orders into thought?
Socrates believed that all learning consists in being reminded of what we already know, and Jerne simply updated his idea to Darwinian thinking: every being is equipped with a library of all possible behavior and cognitive life simply consists of finding (within that library) the behavior that best copes with the environmental conditions.
The genes encode that "library". They encode information accrued over millions of year of evolution.
Learning is discovering already built-in capabilities. The mind knows already the solution to all the problems that can occur in the environment in which it evolved over millions of years. Given a problem, it is only a matter of retrieving the appropriate solution. Indirectly, it is the environment that selects what the mind does. And it is the genes that have restricted what the possibilities are for the mind.
The first chapter is a detailed account of how modern immunology views the immune system: the immune system routinely manufactures all the antibodies it will ever need, and when the body is actually attacked by foreign antigens some antibodies (the ones that best "bind" with the invader) are selected and start multiplying rapidly to cope with the invasion. Neurons exist because it was written in the genetic code. They perform their function no matter what. Interaction with the environment will prefer some neurons over others.
Gazzaniga's aim is to show that the selection process also governs higher mental functions such as language and reasoning.
The phenomenal rate of learning in children could be explained by admitting that children already "know". What they are learning is what is selected through interaction with the environment. Chomsky's universal grammar is an example. Children quickly learn a language because linguistic knowledge is present in their brain at birth and all their brains have to do is pick what is consistent with the specific language spoken around them.
Gazzaniga provides evidence for similar effects on category formation (children have innate skills for categorizing that helps them build the categories used by society).
And he reasons that the same must be true for reasoning: all humans are equipped with some general features that allow for intelligent behavior in our world.
All animals too. The only difference is the role of the "interpreter": the left hemisphere constantly monitors the working of the various special systems and maintains a sets of beliefs. Occasionally, it may prevail over the subsystems and generate "creative" behavior. The interpretere makes humans less dependent on the environment than other animals, it decouples a little bit humans from the environment. This is the weakest part of the book. The tests that are supposed to support Gazzaniga's theory are inconclusive (at best) and plain boring. While he provides a sound description of how the interpreter infers beliefs, Gazzinga does not explain at all how these beliefs would affect reasoning.
Gazzaniga also draws some conclusions about drug addiction, gambling and hypersexuality.