DR. T. PERCY NUNN IN THE BRITISH JOURNAL OF PSYCHOLOGY.—"In the book before us, Prof. Spearman claims nothing less than to have supplied psychology with a body of principles comparable with the first principles of physics. It is a large claim, even though confined to the psychology of cognition, and if substantiated will secure for its author an enviable place in the history of science....A work which, whatever place it may ultimately assume, is unquestionably one of signal importance....A very remarkable and perhaps epoch-making book." THE HON. BERTRAND RUSSELL IN THE NATION AND THE ATHENAEUM.—"A sober and learned, but thoroughly readable discussion....The book is valuable for its wide knowledge, its lucid discussions, and its thoroughly scientific spirit." THE TIMES LITERARY SUPPLEMENT.—"The whole work is excellent, a model of lucid exposition." PROF. C. W. VALENTINE IN MIND.—"This volume gives us in convenient compass not only the essential conclusions of some of Prof. Spearman's valuable researches but a very remarkable additional contribution to the psychology of cognition." THE DAILY NEWS.—"One of the most important works on psychology we have had for some time....A sound and brilliant piece of exposition." THE SUNDAY TIMES.—"A book of the highest value."