The concept of "intelligence" does not have one official definition under the parameters of cognitive psychology. Extensive research in the area continues to bring up new discoveries about neural processes that, as a result, open the door to even more theoretical inquiry about what exactly intelligence is.
As a result, a myriad of theories of intelligence and learning are used as foundations upon which scientific investigations are built, keeping in mind that as the word "theory" implies, constructs are bound to change at any moment when new discoveries are made.
We can go back to the 1880's when two reputable researchers proposed two separate definitions for intelligence. C.Spearman's "G-Factor" theory deems intelligence as a "generalized= G" ability to acquire information and apply it. Using quantitative and purely standardized methods, Spearman concluded that people who "are born with" this general ability will always perform better than those who are not. Notice that there is no mention of specific skills or strengths; in this theory, intelligence is merely a universal and generic ability.
The generalized concept of intelligence fell apart shortly after the G Factor theory, when more tests were applied to individuals where the specific skills that Spearman missed to test were included. This is one of the first instances where "intelligence" is analyzed under a multidimensional scope, and where personal inclination, motivation, and inherent skills are considered. As of 1935, the PMA (Primary Mental Ability) hypothesis was proposed by Thurstone. This PMA model is like a prototype hypothesis that precedes Garner's Multiple Intelligence theory. Both, the PMA and the MI theories concede that individuals do not have a "general" and equal learning ability, but that skills are unique to each person. In the PMA, the basic skills that are tested to obtain data are the same ones we see in the WAIS and Stanford-Binet IQ tests:
- spatial visualization
- numeral ability
- word fluency.
Years later, Sternberg (1985), defined intelligence as
a mental activity directed toward purposive adaptation, tp selection and shaping of, real-world environments relevant to one’s life
This defines intelligence as a cognitive, survival skill which is to be applied to social scenarios in the form of basic problem solving, deductive thinking, and critical thinking. For this reason, Sternberg developed the triarchic model of intelligence where the basic premise is that learning must go through three cognitive applications: a) Analysis, b) creativity and, c) practice. This is where the word "KUD" comes from in cognitive learning: Knowing, Understanding, and Doing are the basic three components of intelligence that build up schema.
The most recent and accepted theory is Howard Gardner's Multiple Intelligence theory, which contends that there are anywhere from 8-13 different ability areas within the human brain which makes some perform better than others in specific tasks. Some of these intelligences (a full list of them is nearly everywhere in the internet) include intrapersonal, interpersonal, musical, verbal, and kinesthetic intelligences.
As you can see not one of these theories adjudicates universality, nor a universal meaning to the concept of "intelligence". Intelligence is a composite of what is learned and how it is applied effectively.