The existence of the "g" factor was originally proposed by the English psychologist Charles Spearman in the early years of the 20th century. He observed that children's performance ratings, across seemingly unrelated school subjects, were positively correlated, and reasoned that these correlations reflected the influence of an underlying general mental ability that entered into performance on all kinds of mental tests. Spearman suggested that all mental performance could be conceptualized in terms of a single general ability factor, which he labeled "g", and many narrow task-specific ability factors. Soon after Spearman proposed the existence of "g", it was challenged by Godfrey Thomson, who presented evidence that such intercorrelations among test results could arise even if no "g"-factor existed. Today's factor models of intelligence typically represent cognitive abilities as a three-level hierarchy, where there are many narrow factors at the bottom of the hierarchy, a handful of broad, more general factors at the intermediate level, and at the apex a single factor, referred to as the "g" factor, which represents the variance common to all cognitive tasks.