Taking the number of components to an extreme is Guilford's "Structure of Intellect" theory. Under this theory there are three fundamental dimensions: Contents, operations, and products. Together these give rise to 120 different categories [Gui67] (in later work this increased to 150 categories). This theory has been criticised due to the fact that measuring such precise combinations of cognitive capacities in individuals seems to be infeasible and thus it is difficult to experimentally study such a fine grained model of intelligence.

A recently popular approach is Gardner's "multiple intelligences" where he argues that the components of human intelligence are sufficiently separate that they are actually different "intelligences"[Gar93]. Based on the structure of the human brain he identifies these intelligences to be linguistic, musical, logical-mathematical, spatial, bodily kinaesthetic, intra-personal and inter-personal intelligence. Although Gardner's theory of multiple intelligences has certainly captured the imagination of the public, it remains to be seen to what degree it will have a lasting impact in professional circles.

At the other end of the spectrum is the work of Spearman and those that have followed in his footsteps. Here intelligence is seen as a very general mental ability that underlies and contributes to all other mental abilities. As evidence they point to the fact that an individual's performance levels in reasoning, association, linguistic, spatial thinking, pattern identification etc. are positively correlated. Spearman called this positive statistical correlation between different mental abilities the "g-factor", where g stands for "general intelligence"[Spe27]. Because standard IQ tests measure a range of key cognitive abilities, from a collection of scores on different cognitive tasks we can estimate an individual's gfactor. Some who consider the generality of intelligence to be of primary importance take the g-factor to be the very definition of intelligence [Got02].

A well known refinement to the g-factor theory due to Cattell is to distinguish between, "fluid intelligence", which is a very general and flexible innate ability to deal with problems and complexity, and "crystallized intelligence", which measures the knowledge and abilities that an individual has acquired over time [Cat87]. For example, while an adolescent may have a similar level of fluid intelligence to that of an adult, their level of crystallized intelligence is typically lower due to less life experience [Hor70]. Although it is difficult to determine to what extent these two influence each other, the distinction is an important one because it captures two distinct notions of what the word "intelligence" means.

As the g-factor is simply the statistical correlation between difference kinds of mental abilities, it is not fundamentally inconsistent with the view that intelligence can have multiple aspects or dimensions. Thus a synthesis of the two perspectives is possible by viewing intelligence as a hierarchy with the g-factor at its apex and increasing levels of specialisation for the different aspects of intelligence forming branches [Car93]. For example, an individual might have a high g-factor, which contributes to all of their cognitive abilities, but also have an especially well developed musical sense. This hierarchical view of intelligence is now quite popular [NBB+96].

## 2.6 Ten Definitions Of Human Intelligence

"Viewed narrowly, there seem to be almost as many definitions of intelligence as there were experts asked to define it." - R. J. Sternberg quoted in [Gre98]
In this subsection and the next we will overview a range of definitions of intelligence