The Genevan and Cattell-Horn theories of intelligence are compared. The theories are found to be similar in the following respects: Intelligence (operative intelligence and fluid ability) is conceptualized as adaptational in function; the products of everyday learning and crystallized skills reflect the impress of experience; one category of intelligence (operative intelligence, fluid ability) is conceptualized as prior or more fundamental than the other (learned products, crystallized skills). Important differences were also found: Whereas fluid ability is characterized as formless and fixed, operative intelligence is viewed as highly structured and evolving; a compensatory relation between noegenetic crystallized skills and fluid ability is hypothesized where such a relation is not conceived to exist between operative intelligence and learning. The relation of Piagetian operative level to the child's capacity to use crystallized solution procedures (aids) in making elementary numerical comparisons was investigated. Performance on quantitative comparison tasks reflecting the child's understanding of correspondence relations was highly related to operative level. It was also found that the child's capacity to implement solution aids in making quantitative comparisons was, to some extent, moderated by his or her level of operative development.