(24pp.) Spearman's notions of mental energy and mental span presage modern conceptions of attentional resources and working memory as fundamental to intelligence. Viewing attention as the conative directing of the intellect, as "the application of intellectual energy," Spearman's quantitative law of mental span deals with limits on the allocation of attention. Because attentional resources are salient in both historical and current conceptions of intelligence, the occurrence of multiple and alternative modes of attention complicates these theories. Moreover, such consistent individual differences in attentional mode have important implications for the theory and measurement of cognitive processing more generally. Specifically, two broad bipolar factors have been identified that contrast sharp-focus versus broad-focus scanning and signal versus information scanning. These stylistic factors are linked to personality and reflect not only the enhancement of information processing in the focus of attention, but also the possibility of parallel processing in the fringe invoking the potential need for active inhibition of distracting or competing processes--points that were also anticipated by Spearman.