required physiological explanation remain to the end undiscoverable, the mental facts will none the less remain facts still. If they are such as to be best explained by the concept of an underlying energy, then this concept will have to undergo that which after all is only what has long been demanded by many of the best psychologists--it will have to be regarded as purely mental. Both by history and otherwise, the concept of energy belongs at least as much to the science of mind as to that of matter" ( 1927, p. 408).
The measurement of g in individuals has been the most problematic aspect of Spearman's contribution and was nearly the sole subject of the first critical review of Spearman most important book, The Abilities of Man. 24 The problem is the indeterminacy of factor scores. They cannot be determined exactly but can only be estimated from the data. In practice, this is usually accomplished by using the most highly g-loaded tests, although even scores on highly g-loaded tests are always contaminated to some degree by one or more other factors, including specificity, in addition to their g. However, these extraneous non-g factors can be reduced considerably by obtaining a "weighted average" of the individual's standardized scores on a number of highly g-loaded tests, in which the individual's standard scores (z) on the various tests are each weighted (i.e., multiplied) by their g-loadings and the resulting products (g × z) are summed (Σgz). This sum is called a g factor score. 25 (It is usually transformed to a standard score, to make it easily interpretable in relation to a particular group or a representative sample of some population.)
Spearman's four other quantitative principles have faded with time or have been supplanted by other terminology and conceptual formulations, so they are mentioned only briefly here, and defined in Spearman's own words. 26
Retentivity . "The occurrence of any cognitive event produces a tendency for it to occur afterwards." This is the basis of conditioning, learning, and memory in its several empirically distinguishable forms.
Fatigue. "The occurrence of any cognitive event produces a tendency opposed to its occurring afterwards." This is akin to Pavlov's hypothetical construct of "inhibition" and is even more closely akin to Clark Hull's postulate of "reactive inhibition."
Conative control. "The intensity of cognition can be controlled by conation." By "conation" Spearman means drive, motivation, will.
Primordial potencies. "Every manifestation of the preceding four quantitative principles is superposed upon, as its ultimate bases, certain primordial but variable individual potencies." This is a recognition of innate individual differences in predisposition to mental development and the evolutionary origins of mental organization.