The Trouble with "Intelligence"
"The word "intelligence" as an intraspecies concept has proved to be either undefinable or arbitrarily defined without a scientifically acceptable degree of consensus. The suggested remedy for this unsatisfactory condition is to dispense with the term "intelligence" altogether when referring to intraspecies individual differences in the scientific context and focus on specific mental abilities, which can be objectively defined and measured. The number of mental abilities, so defined, is unlimited, but the major sources of variance (i.e., individual differences) among myriad abilities are relatively few, because abilities are not independent but have sources of variance in common.
The empirical fact that all mental abilities are positively correlated calls for an analytic taxonomy of mental abilities based on some form of correlation analysis. Factor analysis has proven to be the most suitable tool for this purpose. By means of factor analysis it is possible to describe the total variance of various abilities in terms of a smaller number of independent dimensions (i.e., factors), or components of variance, that differ in their degree of generality. "Generality" refers to the number of abilities that are correlated with a particular factor. The common factors in the abilities domain can be represented hierarchically in terms of their generality, with a large number of the least general factors (called first-order or primary factors) at the base of the hierarchy and the single, most general, factor at the apex.
Ability measurements can be represented geometrically and mathematically as vectors in space, with a common origin and with the angles between them related to their intercorrelations. Factors are the "reference axes" in this space and the number of orthogonal