In contrast, L.L. Thurstone argued that a "g" factor extracted from a test battery reflects the average of all the abilities called for by the particular battery, and that "g" therefore varies from one battery to another and "has no fundamental psychological significance." Along similar lines, John Horn argued that "g" factors are meaningless because they are not invariant across test batteries, maintaining that correlations between different ability measures arise because it is difficult to define a human action that depends on just one ability.