Gardner begins with the example of Franz Joseph Gall, the father of phrenology, or the pseudoscience of attributing personality traits and intellectual capacities to differences in head and skull shape, uniformity, and size. Gall’s research had many flaws, including that brain size has no meaningful implications for success and cannot be successfully correlated with it through research. Additionally, his work was repurposed by racist pseudoscience to imply that different races had different intellectual capacities based on physical attributes. However, Gall was one of the first people to propose that different parts of the brain had discrete functions. Though his other research was disproven, later discoveries proved that particular brain functions are localized and that damage in a particular region affects brain functioning in consistent ways.

The field of psychology has long searched for a neurological location for information processing, but the methods of testing have posed a significant problem. Measuring sensory capacities progressed into linguistic and reasoning capacities in later forms of testing. The concept of reliably predicting a person’s future capacities and successes drove the social fascination with IQ and intelligence testing; indeed, it cemented psychology as a field in which governments and institutions should invest.

However, the imperfect nature of the tests, the results of which skewed favorably toward people who were highly literate, comfortable in an academic setting, and accustomed to sitting still and taking written tests, meant that some people missed out on social opportunities for no real cause. Additionally, even refined versions of the tests didn’t provide compelling evidence for any one model of human information processing.

In this environment, the Swiss psychologist Jean Piaget started to study not the answers to the tests, but the lines of reasoning children undertook when attempting to solve the problems. He realized that IQ tests’ abstract and often random questions advantaged children with a higher facility with language, as well as those who attended wealthy private schools that could afford teachers in the classics. Crucially, they measured how well children retained information, not how well they assimilated new information and solved unfamiliar problems.

In studying the deficiencies in testing, Piaget realized that as children learn, they develop internal rules based on their observations that dictate their future problem-solving. They internalize external information, ordering it in their heads, and then construct symbols that help them retain information and reason with new information. By adolescence, children can freely hypothesize and test their rules of operations, “achieving the end-state of human cognition” (20), according to Piaget. However, this method of intelligence testing, while an improvement on the earlier model, still prioritizes a type of intelligence associated with science that doesn’t mimic or interact with artistic or holistic types of intelligence. 

Another approach, called information-processing psychology and developed alongside improvements in cognitive psychology and computing, attempts to study and replicate intellectual thought so perfectly that a computer can simulate it. While this has led to more understanding of how humans think, computers cannot simulate the complexity of human thought or the interaction between human biological function and consciousness.

Gardner then posits a fundamental shift in intelligence study: using the “symbol systems” approach. This is one of his main fields of study, and as such, he acknowledges that he may have a positive bias toward the system. He states that symbol use, unlike sensory processing or even problem solving, is unique to humanity and thus is the main indicator of human intelligence. Symbols are the basis of myth, language, art, and science and showcase the diversity of human perception; the same symbol can change meaning based on the medium in which it is presented, the person who presents it, and the person who receives it. Additionally, symbols are present in every human culture and can often be understood even by those who do not share the same cultural or linguistic knowledge. As an example, Gardner uses chess playing and the presence of strategic game play in almost every human culture. Though people may not know how to play chess, or equivalents like the Japanese game of go, when encountering people playing chess or strategic games in art, film, or other media, people understand that the game is a test of skill, the results of which determine the relationship between the players. Additionally, symbol usage in music is almost universally interpreted correctly, with people able to identify “sad,” “happy,” or “exciting” music from any culture. Gardner argues that this type of symbol interpretation is one of the best ways to define and study the different types of intelligence.

Symbol-based study has flaws as well: For instance, children who are raised in highly literate settings have predictably different and often more complex ways of interpreting symbols. However, humans everywhere use symbols to communicate, regardless of their education. Additionally, symbol-based research allows for study of the different domains of intelligence, expanding past linguistic-logical thinking and scientific reasoning. Gardner thus proposes that symbol-based systems are the right framework to build upon in studying the true range of human potential.

The two main things that Gardner believes constitute the basis of human intelligence are the flexibility of human development and the identity, or nature, of the intellectual capacities that humans can develop. Scientific and academic progress has ascertained that both factors are much more complex than previously imagined. In Gardner’s view, there is significant flexibility in intelligence, particularly in early development. However, strong genetic constraints can guide development along some paths rather than others. He cautions that intellectual proclivities are not inherited and that the intellectual capacities of a child’s parents do not determine the child’s. However, genetics does determine types and degrees of sensory processing, which may predispose a child to particular types of intelligence.

Though gene research is invaluable in showing heritability for particular diseases and physical traits, no compelling research has shown any genetic component to achievement in higher human expression, like music or language. Gene expression is polymorphic, or able to affect many different aspects of an organism and to affect different organisms in different ways, so genetic heritage is difficult to track and define. However, in the same way that individuals can inherit genetic risk for disease, Gardner states that they can be considered “at promise” for intellectual achievement according to their genes. However, he warns that promise is not the same as talent: Someone with the potential to become a chess champion will never do so if they are never given a chessboard.

The neurobiological perspective, states Gardner, “promises to bear much richer fruit” in the study of intelligence (38). Neurobiology, with a focus on the operations of the nervous system, seems to affect cognition and the mind much more obviously than genetics. A key aspect of neural growth and development is canalization, or the tendency of any organism to follow particular developmental paths rather than others. Neural connections grow in precise ways across organisms, with little variation. While environmental influences can affect superficial aspects of growth, nervous systems grow with the “goal” of becoming an adequately functioning system. This aspect of neurology is canalized, or genetically predetermined. 

However, plasticity, or the ability of biological organisms to develop to suit their environment and needs, is also crucial. Early experiences can fundamentally change the way a child’s brain processes, but its competence at processing is often preserved. As an example, Gardner speaks about a child losing an entire hemisphere of their brain who still learned to speak, while a teenager or an adult who lost the same part would have a permanent disability. Children who are denied particular sensory experiences will develop brains designed to overcome that deprivation. In this way, particular types of learned or canalized behavior, often shaped by precise plasticity, form a “kind of alphabet from which one can compose far more complex forms of learning” (50). Canalized behaviors can predispose a human brain to particular types of intelligence.

Gardner then details the organizational structure of the human brain, with particular emphasis on the differences between the molar and molecular levels of the brain. The molar level involves areas of the cerebral cortex visible to the naked eye, while the molecular level involves single cells within the same structure. As a focus of research, the molar level tracks physical aspects of the brain like trauma from an accident, whereas the molecular level studies how the cells grow and interact with one another. On both the molar and molecular levels, cognitive function can be precisely located in precise regions of the cerebral cortex. Damage to Broca’s area of the frontal lobe creates difficulty with speaking even as the individual still comprehends language fully, while damage to Wernicke’s has the opposite effect. 

Gardner posits that this means that average adult cognitive functions can be linked to specific areas of the brain. He pulls together research from neuro- and cognitive-psychologists, as well as from philosophers, to indicate that human cognition depends on several discrete computational mechanisms within the brain. Some functions, like face processing, are also found in animals, while others are unique to humans, like grammar or syntactic parsing. This “modular” view is characterized by understanding various mechanisms as independent from one another. Gardner admits that this view leaves open some questions, like how plasticity affects the mechanisms and to what degree the mechanisms work with one another.

Gardner concludes by stating that this neurobiological review suggests “a biological basis for specialized intelligences” (60). However, he warns the reader not to underestimate the influence of culture on intelligence and its expressions.

Gardner reflects that the aim of this book is “a question not of the certainty of knowledge but, rather, of how knowledge is attained at all” (63). He warns that the study of intelligence will never result in a single key that solves the question. Instead, it will involve progress, failure, review, intuition, and collaboration, all of which will result in greater insight into intelligence as a human attribute. Gardner states that, though he attempts to categorize types of intelligence in the following chapters, there can never be an unimpeachable master list of all types and forms of intelligence and that anyone attempting to construct such a list would inevitably inject it with their own biases. 

That said, Gardner posits that basic prerequisites for what constitutes an intelligence do need to be defined. He proposes that to be considered an intelligence, a given human competence must be able to “resolve genuine problems or difficulties” and also have the potential to recognize problems as they arise (64), though he acknowledges that the definition of a genuine problem will differ across cultures. Beyond these basics, he details a set of criteria, the majority of which the different models of intelligence must meet to be considered an intelligence. These criteria, or “signs,” include things like the potential for a “genius” or prodigy to exist in the specific field (like music or math), an identifiable core operation or neurological mechanism that develops and can be altered via brain damage, an evolutionary history, and some support from the field of psychology in proving its existence. Finally, and crucially, an intelligence should be able to be encoded in “its own special symbol system” (70), like language, music, or math, so that it can be replicated and shared to some degree.

Gardner concludes by also defining what an intelligence is not: It is not dependent on a single sensory system (e.g., Beethoven was able to continue his musical career even as he developed deafness), and it is not a moral capacity. Any intelligence can be used for good or evil, and no intelligence dictates one’s morality. Intelligence is also not ability; it is in fact more usefully defined as potential. Gardner notes that his objective in presenting this framework of intelligences is more to “illuminate scientific issues and tackle pressing practical problems” than to set forth a new definitive model of intelligence (74).